I looked it up and found the timetable for the ATVO buses that go from the airport into the city. http://www.atvo.it/orari/pdf_airport/mpolovenezia.pdf
As you can see, there are buses up until 00:20am.
Anyway, this is the website for the bus company: http://www.atvo.it/eng/indexen.php
Hope that helps (by the way, I've not been to Venice, I've literally just looked all this stuff up, so can't comment on how efficient this service is!)