vaporetto pass needed?

If you are going over several days before the start of the tour I'd say at least a three day pass is needed for that time frame. Suggest you shoot an email to the RS office and see if a pass is part of the tour in Venice. No need to buy more than what is needed. We've been there a number of times so let us know if you need specific info.

The additional cost of extra days is very low -- and the convenience of knowing you can jump on a vaporetto when you don't want to walk 10 minutes more is very high.

If you are arriving Venice airport, you can also add the airport bus onto the vaporetto pass for an additional 6 Euro. It is normally an 8 EUR ticket. The 7-day pass is 60 EUR (66 with a one-way airport bus ticket) and the 3-day is 40 EUR. So you are only paying an extra 20 EUR for 4 more days. With single ride tickets costing 7.50 EUR each, if you only take 3 more rides over those next 4 days you are ahead.

The tour itinerary doesn't promise a vaporetto pass. While any transport for group sightseeing will be handled by the guide, you would probably still enjoy having your own pass for use during free time, especially at the price of only 20 euros more.

Definitely get the 7 day pass. As Sam said, with each single ride costing €7.50, it's hard for it NOT to pay off, quickly. I found that in Venice, I both walked a lot and took vaporetti a lot; it wasn't "either-or" but "both-and."

Once you have the pass, you'll see how it's a great luxury to take a vaporetto just a few stops to avoid a convoluted walk, or to use the vaporetto to make sure you don't get lost (some destinations are easy to reach from the vaporetto stop but more confusing to reach on land). You can also take side trips to other islands, including the fascinating cemetery island of San Michele.