>> Maybe it's not working since vc-dir-refresh already doesn't use
>> vc-run-delayed to run vc-dir-refresh-files and vc-dir-update.
>
> I don't know if that's the way to determine this. Doing both calls in the
> callback is a good approach which is easy for vc-dir-refresh to
> take. dired-vc-next-action couldn't do that, so vc-run-delayed could be
> the next best option. Or not, of course.

Actually, vc-run-delayed can't be used because it's fired when the first
vc-process finishes, but vc-dir runs at least 4 processes in sequence.

So the only available callback is in vc-dir-refresh, and only when
`more-to-come' is nil.

> Here's a slight modification: create both the hook and a function inside
> the vc-dir package (let's call it vc-dir-mark-files) which will accept a
> list of file names and mark them. dired-vc-next-action will then call it
> from the hook.

All this works fine with the patch below.  The only part of the patch
that I don't like is too much code in the first part of vc-dir-mark-files
that just resolves discrepancy between Dired and VC-Dir:
dired-get-marked-files returns directory names without the trailing slash,
but vc-dir-fileinfo->name returns directories with the trailing slash.