>> I think the only possible problem here is that lambda is invoked
>> asynchronously. But we can sleep while waiting for it.
>
> Would it be possible to call dir-status-files synchronously?

If asynchronousness is inevitable, we could rely on callbacks.
Indeed this path leads to callback hell, but what are other options?

This unfinished patch shows a possible development direction:
vc-next-action executed in Dired will call vc-dired-deduce-fileset
with a callback to continue with a prepared vc-fileset.
vc-dired-deduce-fileset will call a more general
vc-directory-deduce-fileset that will call a currently
unimplemented function vc-dir-status-files whose implementation
will be like in your diff-hl-dired-update.

Also a new command vc-next-action-on-root will commit all modified files,
ignoring any unregistered files.  I miss such a command in VC-Dir:
typing 'v' (vc-next-action) at the beginning of *vc-dir* buffer signals
the error: "When applying VC operations to multiple files, the files
are required to be in similar VC states.  File in state 'edited' clashes
with state 'unregistered'".  Often I have a lot of unregistered files
lying around for a long time, and it takes too much time manually marking
only edited files for every commit, skipping each unregistered file.

Also some parts of the previous patch should be reverted
because I discovered that state changing VC operations in Dired
even on marked files (as opposed to directories) is unreliable:
when a file is modified, using vc-state doesn't return its real state
when updated outside of Emacs.  This part should be replaced
with asynchronous dir-status-files that returns the most accurate state.