I'm guessing it might also depend on the number of flags.
Since this site isn't open source (except for a very old version) we can only guess at how it works. But my guess is it might work like this:
1. Every time someone flags an article, it increments the "flagged" counter
2. Once the flagged counter reaches a certain threshold, it becomes flagged – but vouched is displayed to eligible users
3. If an eligible user vouches it, the "flagged" counter is decremented again – if it falls beneath the threshold, it is automatically unflagged
4. However, there is a second threshold, and once the "flagged" counter reaches that, then nobody can vouch it any more
And that's not mentioning the ability of admins to manually override, either by forcing an article to flagged (so vouching isn't possible), or by disabling flagging on it (so votes to flag it are ignored)
Looks like we're misunderstanding vouches, I guess when we recall seeing a flagged article with a vouch option, it was because it was both flagged and dead, if it's only flagged, there wouldn't be a vouch option since the main counter to flagging is upvoting.
Since this site isn't open source (except for a very old version) we can only guess at how it works. But my guess is it might work like this:
1. Every time someone flags an article, it increments the "flagged" counter
2. Once the flagged counter reaches a certain threshold, it becomes flagged – but vouched is displayed to eligible users
3. If an eligible user vouches it, the "flagged" counter is decremented again – if it falls beneath the threshold, it is automatically unflagged
4. However, there is a second threshold, and once the "flagged" counter reaches that, then nobody can vouch it any more
And that's not mentioning the ability of admins to manually override, either by forcing an article to flagged (so vouching isn't possible), or by disabling flagging on it (so votes to flag it are ignored)