Point of order: I've never heard of a VR SAU, but then again, maybe Wink can educate me, as he often does.
Otherwise, from what I've seen, what Flash said. The preference is to hire from within the community. If that means a CAT II transition, then it's not as big a deal. I've also seen that folks coming from a more "demanding" community makes it easier to go to a less complex mission set, ie going from an ASW platform to a non-ASW platform. I'm guessing going VFA to something not VFA would be similar (within Type, of course).
Also something to keep in mind for upward mobility... The TRACOM SAUs will take non-TRACOM PCOs, however they still want FRS experience at some point in their career. FRS SAUs tend to want FRS experience, as well, so it starts to become a self-licking ice-cream cone.