You do realize that's either a) not vengeance or b) shopped, right?
Vengeance is actually a horde dominated BG with most of the high teams coming from Ysondre (French) and Mug'thol (German).
There's a couple Alliance dominated BGs too, like
Blutdurst, where Aegwynn is just about the only server that has active PvPers; but realistically, most high rated teams are actually Horde - Blackout, Cataclysm, Cruelty, Emberstorm, Reckoning and, most of all, Cyclone all have significantly more Horde teams in their top 10-20 in 3v3 than they have Alliance. It's not about the racials, it's all about what realm dominates what BG and what faction is more represented on said server, everyone will more there regardless so they have more option far as teams and team mates go regardless of racials.