The rule that seems to make the most sense to me is treating the "unarmored belly" as an aspect to be tagged or invoked, no argument there. After it becomes clear that the character is facing a RCV, and assuming he has at some point made the Lore check or otherwise become aware of the blood-bladder weakness, he tags or invokes that aspect (I'd say on a successful hit), to threaten the draining of their blood and the loss of most of their inhuman powers. Badass, important vamps would be able to buy out of the compel, peons would be screwed.
Aiming and Called Shots seem like good mechanisms for doing solid damage and bypassing their inhuman toughness, and I'd definitely use it as a way to even the field just a tad without being forced to spend fate points. I still like having heavier-hitting option too, though, for when things get hairy and you need to incapacitate a RCV as quickly as possibly, like Harry, Michael, and Thomas at the vampire ball. Fate point cost still limits its repetitive use without making it a total silver bullet against Bigger Bads.