Look at my post immediately previous to yours. It links to a Lending Tree study that indicates an inverse relationship between vacancy rate and median home price. Hawaii is an obvious outlier, but interestingly, so is New Jersey. (What's going on in Jersey?) The data in this study is broken down by state; SMSA would probably be better, but I didn't spend a lot of time digging up data.
That data backs up what I am saying though. The national average means nothing, even state averages means nothing if where you are looking for housing, the vacancies are either not high enough to facilitate moving people around or are in conditions or floor plans that do not fit your needs. Not every vacant unit of housing is equally fungible.
California having one of the lowest vacancy rates and one of the highest median housing rates means there potentially needs to be a much higher rate of vacant units to lower housing prices if they are as inversely related as you describe.
