There's a "vouch" button on flagged comments and some flagged stories, which will unflag a story. I'm not entirely sure under which conditions it's available on flagged stories (it's almost always there for flagged comments). I think it probably gets disabled after it got flagged → vouched → flagged, or something? You need at least 500 (or more?) reputation points regardless.
Comments
There's a "vouch" button on flagged comments and some flagged stories, which will unflag a story. I'm not entirely sure under which conditions it's available on flagged stories (it's almost always there for flagged comments). I think it probably gets disabled after it got flagged → vouched → flagged, or something? You need at least 500 (or more?) reputation points regardless.
I have >1800 reps and I've never seen that
It's only on [dead] items, not [flagged]. Sometimes it's [flagged][dead] but the only thing that matters for allowing to vouch is if it's [dead].