https://news.ycombinator.com/item?id=10298512 Maybe that will answer your question. But just exercise common judgment, think of a possible reason as to why a particular thing was flagged [dead] to begin with, and then determine if it's unfair.
I've been able to consistently save dead comments with a single vouch, so I presume it only takes one, at least for some level of HN user.
FWIW, I've always treated the "vouch" feature as something to be used fairly infrequently, for somewhat exceptional cases. I don't know if that's actually the intent or not, but it "feels" like the kind of thing one shouldn't be doing all the time. shrug