I have had the same type of issue with vent,my mike would work before I logged into the game but once I was logged in my mike was dead.
I had the mike set for push to talk and it wouldn't key up in game,when I set it to voice activation it would work.
I then went to this site
http://forum.ventrilo.com/
and looked up the issue and found that if I set my talk key to something like capslock vent would see it and this did work.
it does screw up my spamming /C all the time so I have to remember to check if the caps key is on LOL