With the exception of Stephen Caldwell, we arguably haven't had a player on our roster for two seasons who could start at center back on almost any team in this league. If Perquis isn't good enough because he can't be consistent (and hasn't been for most of his career, same with Kantari) why did they expect him to become so in MLS?
There are lots of players who appear, in the odd game, to have enough combo of technique and intelligence to succeed but really don't because of inconsistency. Rohan Ricketts, Martin Saric anyone? It's the hallmark of the MLS 1.0 player, the Euro journeyman etc etc Every team in MLS has them; the one with the fewest in the starting lineup tends to win.
I'm by no means saying Vanney has been perfect, but when you look at the shuffling he's done this season as Og has pointed out, it may also be that our options are just kind of shit in that area. When players don't perform, you can't by default say "the coach is at fault." Often, the player just isn't good enough. In fact, as Taylor Twellman pointed out the other night, we've been in the bottom six of the league defensively in EVERY season in the league. Look at where those central defenders have gone; it's the one position where not one former TFC player outside of perhaps Marvell Wynne has made a contribution elsewhere in the league.