How to buy a monthly vaporetto pass

Here is a site describing the "regular users" Venezia Unica card. It costs 100 EUR, good for 5 years. Then you load money on the card and get the resident's price of 1.50 EUR per vaporetto ride. Looks like you need an ID photo to go with it.
https://europeforvisitors.com/venice/articles/imob-cartavenezia.htm

The Venezia Unica card is a good deal if you plan to do a lot of travel, and especially if you plan to return, because it's good for several years (5 I think). It may be a better deal than 4 separate week unlimited tickets, but it does have a per-ride cost, so it depends on how many rides you plan to take. Mine cost Eur 100, that may still be the price for non-residents of the area. It also gets you a discount on some other transportation, like the Alilaguna to the airport. I got it at the Piazzale Roma ticket office, I think that may be the only place you can get it. Don't go to the outside ticket window (closest to the parking lot), go to the inside office at the other end of the building. They'll take your picture and issue your card there. I think you'll need your passport. You can add vaporetto tickets to it, either one at a time for Eur 1.50, or 10 for Eur 14.00 (Eur 1.40 per ride). It used to be you had to go to a ticket office to add these (any of the bigger staffed ones), but the last time I was there, it looked like they were setting up ticket machines at more stops to be able to add these for cardholders.

Thanks both of you for pointing me in the right direction. I keep reading about it without gaining a clear basic understanding of how it works. I’ll be staying 26 days and like to not have to worry about buying the 7-day tourist pass. I’ll be staying in the southern part of Connereggio. I actually like the south side of Dorsoduro facing the canal of Giudecca as well as the part around the Guggenheim and Salute but found an apartment I like on the Grand Canal.

The monthly pass is for a calendar month, not four weeks. Available to residents only.
Maybe a Venice Unica Card works forcyou.

I like Dorsoduro also. Cannareggio apartments tend to be a bit cheaper, but they're also not quite as close to the places I like to visit. You may do more walking or vaporetto riding from there, but with a whole month you should have plenty of time to see things (I don't know your itinerary or interests). Reading up on the Venezia Unica pass, they're available from several places, but it looks like the only one inside Venice is at Piazzale Roma. If you think you might go back to Venice sometime later, I might encourage you to go for the Venezia Unica card now. (Confusingly, there's also a Venezia Unica visitor card for tourists that includes entrances to museums, etc.; that's not the same thing).

I've done several long stays in Venice, up to 2 weeks at a time. On my first trip, I wasn't sure when I'd return, so I saved myself a few Euro by buying a weekly ticket like you're considering. I would have been better of just getting the card Venezia Unica card on my first trip, because once I have that, it's much more convenient, and inexpensive, on future trips. It's also allowed me to take side trips out of Venice, or just hang out near my apartment without feeling like I'm wasting days on a weekly pass. My card currently has a few rides left on it from my last trip (I've always added 10 rides at a time even if I end up with a few left over), so I can hit the ground running if I go again. Your remaining rides will stay on the card for 5 years. It's renewable too (for another 100 Euro fee). I haven't had to do that yet, so I don't know if rides left on the old card transfer over, but I suspect they do). If I arrive at the airport and want to take the Alilaguna, I can get a cheaper fare into Venice simply by showing the card at the Alilaguna ticket office by the dock at the airport. Otherwise, your initial trip into the city will be at full price.

When you go to the office at Piazzala Roma, go to the machine to take a number, and wait for your number to come up on the screen. I had to wait a while when I did it. That office also handles other services that are mainly for residents, and you may have to choose what service you want. Once you're called, they'll take your picture (no charge) and issue your card on the spot. You can also purchase some rides then too. I'd suggest starting out with 14 Euro for 10 rides. Each time you go through the gate to board a vaporetto, the machine will tell you how many rides you have left. I've added rides at other places with an attendant like the Rialto and Fondamente Nove stops (you may have a wait if there's a line). As I mentioned, on my last visit, I noticed as I was leaving for the airport that a ticket machine at a smaller stop seemed to have a new option for adding rides from the machine, without going to a ticket window at one of the main stops with an attendant. Supposedly you can also add to your card online, but I haven't been able to figure out how to do that. ACTV's website isn't the most straightforward.

So Gary are you saying to go for the monthly pass that costs 100 euro just to obtain and then to add money or rides onto it? Is it a physical card? Thanks so much for sharing this with me. I was getting so confused and tired trying to figure out what it all meant, while going back and forth between sites, to try “to put it together” and be somewhat confident that I understood what they meant.

The 100 Euro Venezia Unica "frequent users" card isn't a monthly pass at all. You don't even get any rides for just that 100 Euro fee. What it does for you is get you resident rates instead of the rates that visitors usually pay, and it's good for any rides you take for the next 5 years. The single ride rates are more than 6 times the rate for frequent users and residents, so if you ride a lot, it can work out to cost you less. The Venezia Unica card is much cheaper, or free, for residents of Venice, but people who are not residents of the region can get one for 100 Euro. Which is better all depends on how much you plan to use the transport services (Vaporetto, Alilaguna, etc.).

Let's do some math.
For a 1 month stay of unlimited vaporetto rides, you could buy four 7-day passes for 65 Euro each, or 260 Euro.
Or, you can buy the Venezia Unica card, and buy 10 rides for 14 Euro. So:
10 rides would cost you 100 Euro for the card, plus 14 Euro. 140 Euro.
20 rides would cost you 100 Euro for the card, plus 28 Euro. 128 Euro.
...
110 rides would cost you 100 Euro for the card, plus 154 Euro (14 x 11). 254 Euro.
120 rides would cost you 100 Euro for the card, plus 168 Euro (14 x 12). 268 Euro.
110 rides at the single ride rate of Eur 9.50 each would cost you 1045 Euro.
If you're planning to do more than 4 rides a day for an entire month, 4 weekly unlimited passes might work better. Otherwise, the Venezia Unica card works better. Plus, the Venezia Unica card gets you discounts on other service, like the Alilaguna to the airport or around the Venetian lagoon, and if you return to Venice within 5 years, bring back the card and you just pay 14 Euro for 10 rides, or 1.50 Euro for one (which you have to preload onto the card before you ride). You won't have to pay that 100 Euro fee again.

The Venezia Unica card is the size of a credit card, and it has your picture on it. It's only valid for you, nobody else can use it (attendants who may check your ticket can see that it's yours). You simply tap it at the gate before you board the vaporetto. If you're traveling with someone else, they need their own card. There's a picture of it at this link: https://europeforvisitors.com/venice/articles/imob-cartavenezia.htm (which was linked above by someone else). This page has info on other fares, and says there's now a tap to pay option with your credit or debit card. https://europeforvisitors.com/venice/articles/venice-vaporetto-fares.htm The system will figure out whether the individual ride or multi-day pass rate is better for you, and apply that rate. I haven't used that, but those credit card tap to pay systems are becoming more common in various cities. More convenient than buying a ticket if you're an occasional use visitor. You could just do that and not buy any other tickets or pass. But, that tap to pay credit/debit card system won't get you the Venezia Unica card fare. That article also shows the ticket machines. I believe that red print and tap card reader in the lower right corner is new, for adding fares to the Venezia Unica card.

Digging deeper into that website, they have a page describing how to get a Venezia Unica card. It's basically the process I described, with some more details:
https://europeforvisitors.com/venice/articles/imob-how-to-buy-a-cartavenezia-card.htm

The Venezia Unica card can also hold monthly and annual commuter passes. Most of those seem to be for people commuting from the mainland. There's one here I'm not sure you'd be eligible for if you have the card:
https://actv.avmspa.it/en/content/venice-urban-services-0
Under the heading Rete Unica there's this:
MONTHLY PASS FOR ISLAND RESIDENTS 21 Euro
Includes all waterborne routes.
The Venezia Unica card gets you the Rete Unica price for other tickets, so I wonder if it gets you this too. This might mean calendar month, not 4 weeks, and I'm not sure what islands it includes, and I'm not sure if you're restricted to a designated commuting route with these passes. I don't know the answer to this.

I bought the Venezia Unica card in 2024. My experience was as described by others, but I had a very, very long wait. There was a lot of movement of people on and off the counter; I suspect I hit the period when people were taking lunch breaks.

Using the card was very easy during my 2-week stay. I'll be back in Venice for about 10 days this September and will just add 10 tickets to the card.

My only regret is that I didn't buy the card on the first of my recent trips back in 2022. I don't ride the vaporetti very frequently, so even the 65-euro weekly pass (though cheaper than individual tickets) isn't a great deal for me.

I came here to help, but after reading Gary’s replies I have to give him kudos for the detailed answers and calculations. Enjoy your stay in Cannaregio with Grand Canal views, Denny!