There are many car rental Companies in Venice that operates transfers to and from Venice with car or bus with driver.
A Company with a good service is Mattiazzo (www.mattiazzo.it
If you want to save money there are some independent drivers with regular license for the transportation of people. You can easily recognize them because behind their car you can see a plate with the inscription "NCC", that means "Noleggio Con Conducente" (the translation is "rental with driver").
I know a guy that speaks a good English and own a big car with a lot of space for your luggage. Try to ask him if it's available for your transfer, you can send him an e-mail to email@example.com
Hope this helps.