With vim, how can I use autocmd's for files in subdirectories of a specific path?

So, I've done this in my code with paths under a particular path. You need to do:

autocmd BufRead,BufNewFile */somepath/* set filetype=sometype

At least, that's what I was using to set a given filetype for things under a particular library path. Hopefully that will help with your example.

If you want to include all subdirectories, use **:

autocmd BufRead,BufNewFile */somepath/** set filetype=sometype

VIM's inbuilt help will answer all your questions. :)

:h autocmd-pattern