> I don't say it's a bad thing, just will make the patch 10 times longer.

Ok, here's the patch that is 10 times longer.

It creates a new function vc-use-vc-dir-on-files
called from vc-next-action that checks whether
the buffer is Dired with marked directories,
and for optimization returns a list of files
to give to vc-dir via a new arg MARK-FILES.

Then vc-dir let-binds the global variable use-mark-files
exactly the same way as already let-binds use-vc-backend
since there is no other way.

Then vc-dir-refresh puts all marks on files.

Also the patch adds a new useful global keybinding 'C-x v V'
bound to vc-next-action-on-root to open *vc-dir* buffer where
all registered files are marked.