When you ask the hunter who this Maiden Fel is, he says she is the reason even monsters have nightmares. This doesn't sound like someone who a vampire lord would hire, even through intermediates. I'd expect him hire someone like the Zhents, who'd deal with a vampire willingly, no need for masquarade.
Astarion is implying the Gur was acting indirectly on Cazador's orders, but he could be trying to misdirect the PC here. He is not willing to confess what exactly he did to the Gur in the past. Maybe he realizes at this point that the Gur could also be after him because of that past, and he doesn't trust the PC with this.
There's a lot of speculation here that I don't see a need for. Astarion's explanations make sense; there's no reason for him to be evasive and no indication that he's trying to be.
Astarion speculates that Cazador hired the Gur for this job because he wanted to send a specific message, which seems reasonable enough. Maiden Fel is no doubt very good at her job - as anyone running a group of monster hunters should be - but that doesn't mean Cazador is afraid of her, or isn't capable of outmaneuvering her, being a centuries-old vampire lord and all. Also, it seems pretty far fetched that the Gur would spend two hundred years magically monitoring a guy who did them wrong once upon a time just on the off-chance they could eventually catch him while away from his master's protection. That's a lot of generations to hold a grudge, and a lot of resource investment for a small and uncertain payoff. Astarion isn't even a particular threat to them now, no more than any other vampire spawn.
