If there is evidence, you should be able to find it pretty easily via Google.
I don't know of any complete compilation of data, but Cheverus posted recently on an article in Catholic World Report lising the American dioceses in order of number of vocations per 100,000 (I think) Catholics. He said it wasn't on the 'net, and the only thing I remember is that Boston was 162 out of 174. (Yes, I'm in Boston.)
If he could identify the back issue, you might be able to get a copy.