Individual vaporetto tickets are exorbitantly priced at 9.50 euros (over $10). Assuming you're going to Venice in order to see sights in Venice (which are mostly not on the Lido), you'll have a minimum of six rides per person, just getting to and from the Lido, unless you use the Alilaguna upon arrival (which sounds like a it would be smart). There will be at least eight rides if you get to Venice early enough on May 25 that you want to leave the Lido that day, again assuming vaporetto rather than Alilaguna on arrival day. So let's call it 5 trips or 7 trips. Again, that's the minimum. Most people like to take at least one trip down the Grand Canal, and you may want to take other vaporetto trips. Going to Murano or Burano will mean two additional trips, or three if you go to both islands on the same day (not necessarily recommended with as little time as you have).
A vaporetto pass will probably save you some money--perhaps quite a bit of money per person, depending on your sightseeing plans. Passes are priced as follows:
24 hours: 25 euros
48 hours: 35 euros
72 hours: 45 euros
168 hours (one week): 65 euros
Although I don't know what time you'll be arriving in and departing from Venice, my guess is that the 72-hour pass will cover you for your entire visit. You can buy vaporetto passes at the train station or at most busy vaporetto stops. I'm nearly certain there's a place to buy them at the airport, too. I'd want to get my hands on a vaporetto pass as soon as possible, because there are a few vaporetto stops where it's impossible to buy any sort of ticket, which means you have to wander off in search of another stop with a sales booth or a vending machine. With all the canals and bridges (which aren't always where you'd like them to be) in Venice, nothing is as close on the ground as it looks on the map, so you can waste a lot of time seeking another vaporetto stop (voice of experience).