A few folk have asked about playing Vanguard in BB's, so here is what I know:
Vanguard skills like Rending bark don't stack, so generally 1 vanguard is enough per instance.
Vanguard is only really needed for 6 man Pel., 3 man DoMT, 6 man Hammer and 12 man Wall.
Vanguard is less important with a high DPS group, generally these groups can burn thru the mobs fast enough even without a player in vangaurd spec.
You can switch between specs as needed, but changing out of vanguard will cost you some kill streak.
Finally, I am posting a screen pic of the vanguard spec I use. I found in on the web and it seems very effective to me, allowing for decent engineer traps while giving you full kill streak and use of rending bark.