Sorry, but I'm gonna go ahead and say right now that neither of those classes are capable of pulling that much in a single hit. As a Shadow I'm guessing that amount came from a double project that counted as one hit, I don't know how the Vanguard achieved that hit.
Not true i have hit +8k backstabs whitout min/max gear or sentinell buff
Im sure whit min/max gear + sentinell buff + someone whit horrible gear 10k is true
I think mine is 8,5k best atm and like i did say not whit the best gear or buffs