It's for flagged comments. If you click the timestamp link, it takes you to a view of just the comment where you can 'vouch', which is a statement you think it is unfairly or inappropriately flagged.
Ah ok thanks. So comments but not stories I guess.
Stories too - all the other flagged dupes at the time showed the vouch option while this one didn't. I suspect this thread got auto-moderated as spam and therefore vouch wasn't available since it could be used to circumvent the moderation. Whereas the user-flagged stories were all vouchable.