You probably saw this: https://news.ycombinator.com/item?id=10298512
That doesn't explain exactly how vouching works, and I think the "strength" of a vouch might be changed per-user (it no longer seems to do the same thing it used to, for me, at least).
I read them over, nothing looked overtly spammy or troll-ey, so I vouched for this comment as it seemed genuine.
I do think it's an anti-spam or anti-troll measure, but it looks like it's a mishit in this case.
That being said, is there anyone where we read up on what vouching/dead/etc... means on HN? Maybe one of the mods or someone can help explain some of it (or let me know that they won't explain it for anti-spam purposes...).