I would say the best way to do this is to treat it as a Called Shot or Painting a Target(YS209). You place an aspect on the vampire representing attacking the abdomen and then you just tag it to hit the vamp in its weak spot 
And in case of a Called shot the vamp can remove the aspect with an Endurance roll( or some other skill).
Called Shot is easiest. You say "I'm cutting his tummy", do the maneuver and it gains "stomach cut open" which you can invoke for effect to make them stay out of the fight. Now, the GM sets the difficulty here, so against a really skilled RCV this might not be remotely practical to do. Should work great for goons though, as least if your attack abilities are high enough.
