Personally, I've never seen a dead comment come back to life. I'm not sure using the vouch button does anything. (It does for submissions, though.)
Personally, I've seen two - both ones that I emailed about (and I really disliked the stance taken on one of those, but they were debating at a level that was polite, addressed objections raised, acknowledged other PoVs etc.)
Vouch, I've perhaps seen reverse things several times (more of a background observation not pursued, so, you know, fuzzy anecdata at best), I did get feedback from @dang that one comment I vouched for was, in fact, a legitimate AI gen comment .. so clearly I'm human enough to fall for the occasional clanker.
I have, but it's definitely very rare. And might have been manual intervention as described above.