I have to admit, I was thinking about this almost entirely from the perspective of a PC with Evocation using it on NPC vampires, not the other way around. From that perspective, where the NPC's internal story doesn't matter all that much and they can be assumed to kill somebody off-screen to empty their stress track, it didn't seem like a big deal.
I thought hunger track stress didn't take away powers on its own, though. Only special Consequences taken when you can't take more hunger stress does that.