'vouch' links only show up on [dead] posts, and [dead] and [flagged] are not the same thing. A post can be both [flagged] and [dead], in which case there will be a 'vouch' link—but not when a post is [flagged] but not [dead].
This makes sense once you understand that [dead] means "not generally visible"*, and the purpose of 'vouch' is to restore a post to general visibility.
* that is, they're visible, but only to users who have 'showdead' turned on in their profile.
Is it correct to conclude that [vouch] links only appear on [flagged] submissions that are below the flag threshhold for [dead]. Maybe three flags or something.
Are the [vouch] rules the same for both submissions and replies, i.e., comments. I inadvertently used the term "flagged comments"; I was actually curious about flagged submissions.
For instance, as I type this there is a submission in /active that's [flagged] but not [dead], but there's no [vouch] link.
This makes sense once you understand that [dead] means "not generally visible"*, and the purpose of 'vouch' is to restore a post to general visibility.
* that is, they're visible, but only to users who have 'showdead' turned on in their profile.