vc-ignore-dir-regexp

vc-ignore-dir-regexp is a variable defined in `vc-hooks.el'.
Its value is

"\\`\\(?:[\\/][\\/][^\\/]+[\\/]\\|/\\(?:net\\|afs\\|\\.\\.\\.\\)/\\)\\'"

Documentation:
Regexp matching directory names that are not under VC's control.
The default regexp prevents fruitless and time-consuming attempts
to determine the VC status in directories in which filenames are
interpreted as hostnames.

You can customize this variable.