Here is info in a somewhat related post from another FlyerTalker - ChgoBob:

"4 choices here.

1. Private launch pick up. Pretty expensive.
2. Taxi to P. Roma, then vaporetto.
3. Bus to P. Roma (free with vaporetto pass, Venice Card), then vaporetto.
4. Alilaguna from VCE to Venice. (also available with the Venice Card or separately)"

We just returned from a trip to Italy and took the vaporetto right in front of the train station, #52 vaporetto stops at Fondamente Nove. It takes a nice litte tour around what I believe is the north side of the island before docking. That was the least expensive and most convenient way to get around the island if you want to see the island and get off your feet.

Here's the Venice Card .