What SemiJ says is true... and regardless of your mainsheet setup, a powerful, effective boom vang is one of the more important sail controls one should have.
I shudder when I see people sailing around with no vang whatsoever - and when I point it out the usual response is "we don't race so we don't need one"... Hogwash. Even just a fixed strop as a vang is better than none at all for downwind work.
But a good vang will compensate for a short traveller, and permit proper main leech tension whenever the mainsheet is eased out beyond the traveller limits.