Shadow vestals with a f9/s9 bless. Twist of fate and MR 19? Good luck with the banishment. Bandar log has access to level 1 priests. ooh . . . I am sure those vestals were shaking in their boots.
Shadow vestals are also a lot cheaper than van (in terms of both gold and resources) so there are a lot more to fight.
The counter for vestals with a bless like this is the same as for the vans - area effect magic. If you try to get by with priests, chaff, massed archers, and the like, you are going to have disappointing results.
In short they are every bit as likely to steamroll MA Agartha as Vanheim is. At least early on. Of course that isn't saying much, as most nations have an easy time picking on MA Agartha in the early game.