That's never happened to me. Once I'm dead I'm dead. I sometimes get my satisfaction later on, for example when Bitcoin was popular on HN I was one of the few people early on to call it a waste of electricity. Now that's not so controversial.
But the vouch button does exist, I wouldn't be surprised if it didn't work for vast majority of dead comments, since they probably wouldn't be dead in the first place if enough people upvoted them.
I guess I don't have enough karma to have a vouch link. The comment by bombcar in this thread is dead and there's no vouch on it.
I think you need to click on the comment timestamp to view the comment in isolation for "vouch" to appear.
Ok, now I see it. Thanks, learn something new every day
Vouching does rely on someone to take the extra effort relative to a downvote, so I think it's safe to assume vouching is more rare.