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.

True, but I'm wondering if the catch is almost too easy. That, and the feeding dependency rules are alright for a player controlled character, but when it comes to draining an enemy NPC, tallying everything up after a scene just doesn't cut it.

My character's High Concept revolves around battling Red Court vamps, and as such many aspects of his character are centered on specializing him to fight them. I was wondering how the Reds' "unarmored belly" catch would be used in actual gameplay. Simply stating "I'm attacking their stomach to meet their catch" seems more than a little overpowered, since any successful attack would essentially render them cut off to most of their supernatural powers.

The only thing me and my GM have managed to come up with is, taking into account that these are supposed to be reasonably badass individuals and shouldn't be able to be slaughtered in the dozens, is a stunt that grants the character the option of, after a successful Weapons hit (tailored to his personal weapon preference), spending a fate point to slice the blood-bladder and deprive the vamp of more or less all its Inhuman Toughness, Strength, Speed, etc. The required fate point ensures that it can't be over-employed, making sure each Red Court doesn't become a total pushover.

If anyone else has other ideas on how to work their "unarmored belly" catch into gameplay rules, I'd appreciate the input.