Only you can determine if a vaporetto pass is good for you.
They come in various denominations. For example a 24 hour travel card is 20€, a 48 hour travel card is 30€, a 72 hour travel card is 40€, for unlimited trips during the time of validity (the time starts from the first validation stamp).
If you pay as you go, each individual trip is 7€ (One hour validity, you can change vaporetto but not reverse direction of your travel).
So it depends on how many trips you intend to take. Certainly I would buy a pass at least for the day you go to Murano and Burano, because that day you will take at least 4 rides, i.e. more than a 24h travel card (7€x4=28€>20€ day travel card). If the next day you take another 2 trips, then a 48h for an additional 10€ will be worthwhile too. If you arrive early and travel to Murano, Burano on the first day of arrival, it might pay to have a 3 day pass too. If you pack most of your tripping on the first 24 hours, then for the additional days all you need is two trips a day and the pass pays itself.
Does it show I'm an accountant?