I interviewed at both and am waitlisted at both. My bias is that I do want to go to UCI but I'm giving my opinion as unbiased as I can.
From my limited interview experience it seems to me that if location (wanting to be in Cali, perhaps proximity to friends and family) matters to you then that is the only reason you might consider choosing UCI. This is the case for me. That coupled with the fact that I do not want to get involved in academic research/medicine and don't have a specialty choice of derm, plastics, ect.
I really echo everyone else's thoughts on this board that Vandy is just in a different league and is one of the top schools- even just doing "average" there you will have no problem coming back to Cali and landing a GREAT residency. Hell if you just get within their average board score which is something INSANE like mid 240's, you will be a rockstar. If you have a really competitive specialty in mind I think Vandy would be a better platform even more so if you were to take advantage of all the school has to offer.