Vaparetto from Marco Polo to Venice

Look at the Alilaguna description on this link.

https://quickvenice.com/transportation/airport-marco-polo.htm

You have to add the No 5 airport bus onto the pass when you buy the pass. The bus goes from Piazzale Roma to the airport (and back). The cost is 6 EUR each way. so if you only need a one-way, you add 6 EUR, or 12 EUR if you need a round trip.
http://actv.avmspa.it/en/node/775
Click on "Time Limited Tickets" for the description.

The vaporetti do not go to the airport. That service is provided by the Alilaguna, a separate company. The vaporetto Pass does not include the Alilaguna.

You can view this map to see the Alilaguna stops around Venice:

https://www.alilaguna.it/en/lines/lines-map

Or you can take a bus from the airport to Piazzale Roma in Venice, and ride a vaporetto from there. That vaporetto ride would be covered by a pass. There is a way to pay a bit more to add the bus link, but we have not done that so I do not know the details.

We just took a cab from Marco Polo. It's not really very far. I was thinking the vaporetto was 15 Euros each, and then you'd have a hike to the terminal.

Don't confuse "vaporetto", bus boats operated by the ACTV for transportation within Venice, and the Alilaguna boats which provide scheduled service between various spots in Venice and the airport. They are separate entities and tickets not interchangeable. ACTV also operates scheduled land buses on the mainland and Lido.

"We just took a cab from Marco Polo". It that will only get you as far as Piazzale Roma, just like the ACTV bus. Unless your hotel is right there, you will still need a vaporetto to reach the hotel.

Yes, you've been given directions for several different options, and they're easy to confuse.

Way number one is to take the Alilaguna boat from the airport directly to Venice. There are several routes, and limited stops in Venice. This takes some time, and reports are that the windows are fogged with salt spray so you get no view. But, it's cheaper than a water taxi, and you don't have to transfer (as long as your boat goes to a stop close to your hotel).

Way number two is to add a bus to your vaporetto pass. There are, in turn, two different buses. Bus number 5 is run by the same company as the vaporetti (ACTV), and can be added to a vaporetto pass. It makes a few more stops. There is also a bus run by a different company, ATVO. This is a separate ticket and makes fewer stops.

Either bus gets you to Piazzale Roma (the furthest into Venice that wheeled vehicles can go). You then walk or (more likely) take a vaporetto to your final destination.

As said above, a land taxi from the airport only gets you to Piazzale Roma.

Yet another option is a water taxi right from the airport. This is much faster and easier than other methods, as it can use any dock in Venice, not just the Alilaguna or vaporetto docks. It's something like €120 per boat (not per passenger), so if you're splitting this among a group, it's not a bad deal.

If you say which hotel you're staying in (or where your other accommodation is, exactly), people can give the best advice on airport transfers. Also, look at your hotel's website; many of them have very precise directions, although if it's a more expensive hotel, they may not discuss the bus plus vaporetto option.