Either this is interesting or I'm doing something wrong

This seems to be a good time to remind everyone following this thread that I have no expertise whatsoever here. I work with numbers for a living and I've lived in California for a while and I've made a real effort to explore the state, but other than that, my knowledge of this topic is limited to what I could glean from following the news and doing a few searches on Google, Wikipedia and Census.gov.

My lack of expertise makes me especially nervous when I come across something that is, for lack of a better word, interesting.

For instance, if I am looking at the right numbers, the vacancy rate for SF is and has long been much higher than San Jose or Oakland (or, while we're on the subject, LA or Fresno), despite having the highest housing prices. 

Other places I've checked follow the expected and presumably causal pattern of shortage, low vacancy, high prices (for example, I get 4.5% for Fresno). Only SF seems to break the pattern. 

Are these number the best way of tracking vacancy? Am I getting the math right? Take a look at the Census data for cities you're familiar with. Are they behaving the way you'd expect?