For Revenants, no, but you've got to work for it. Revenants start off both living and undead:
Undead: You are considered an undead creature for the purpose of
effects that relate to the undead keyword. You are also considered a
This means that for a Revenant the Aura is neutralised at best, until you take the Death's Blessing feat (Dr376).
Benefit: You no longer need to eat, drink, or breathe, so you never
need to make Endurance checks to resist the effects of starvation,
thirst, and suffocation. You are no longer considered a living
At this point, you're about equal to how the Vampire class starts out, for the purposes of this 'exploit'. However as mentioned all Vecna has to do is notice your PCs are still looking pretty healthy after a couple of rounds and use a minor action to turn off the aura (this is specified in MM1's Glossary).
Did they beat the system?
The key thing to remember as a DM is you are the system. This really comes down to whether you want to reward your players for metagaming or not. Pull a M. Night Shyamalan, and have some other Big Bad your players weren't expected come in as the real BBEG. Maybe have Vecna treat the PCs as allies initially - after all, they've paid a heavy price to become undead in order to face him. Make whatever it is memorable - this is the final confrontation after all.