Sounds a bit silly, but are you pressing down the "talk" key? I thought my game was broke once as it reset the spray tag key.
Is it selected as the primary audio input in Vista?
Is your connection speed too slow? On some occasions with a ping of 200+ my mic has cut out.
Theres an article in todays Computer Active that says Vista handles audio differently to XP (which is why Creative or some folks have "fixed" a problem with it). Might have something to do with it?