Maybe a way to handle it "right" is for vertico to register an
`after-change-function` so as to detect also when the buffer's content is
modified outside of the normal commands (i.e. from a timer or process
filter). It'd probably be a bit messy, tho: it wouldn't want to refresh
the completion list right away, but there's no "obvious" later hook to
use (like `post-command-hook`) so it would need to detect when there's
not going to be a subsequent post-command-hook (i.e. when we're in
a timer or similar) and then probably fire its own timer to update the
completion list when the current processing is done.
Or maybe it should use `after-change-function` to detect changes and
`pre-redisplay-functions` to update the list of completion.