It appears that many of the flights out of Venice are early morning. Like 7:30 am. How hard is it to get from your apartment or hotel to the airport in time?
For us the best option was to pre-arrange a boat taxi to take us (the night before), though expensive it worked very well.
Your hotel can help you. Pretty cool way to exit the city too.
Buses start running from Piazzale Roma at 05:00 arriving at Marco Polo at 5:30 The vaporetto system will be running although on a reduced night schedule to get you to P le Roma. The Alilaguna shuttle boat has runs from that will get you to the airport by 05:00 or 06:00