Deleted answers including comments and votes on them will be visible to moderators and users with 10k reputation. But it won't be visible who voted. And if the person who commented cannot see the answer anymore, they may never know that their comment got a positive vote.
But it is still worth upvoting their comment, after all it only takes one click to do so.
Writing a reply using
@username as pointed out by @Reaces will trigger a notification to that user. So you could write something like:
@username my bad, I'll delete this answer.
I'm not entirely certain if you then need to wait for them to see the comment before you delete the answer, or if your response remains visible in their notifications, even if the answer it was attached to is long gone.