Yep. It's like Pete says. Compared to game sounds - Vent's output level is quite low.
What I do is turn my overall PC volume right up to eleven, so the game sounds are WAY too loud... Then, in the game's audio settings, turn the in-game sounds down until they are at the volume I want them. The sliders end up being quite low - around 5% - but because the PC's volume is maxed out, they are actually at the right level.
Now, when using Vent, I have some headroom in the Ventrilo balance.
It's a shame there isn't a better way to balance it - but this way works.