You can vouch for posts, but only once they're [dead]. [flagged] is not [dead] on its own, and so nothing can be done by users yet (other than maybe reaching out to the mod). If it acquires enough flags to become [dead], then those of us with enough karma could vouch for it.
Note that the same thing is true for comments. Occasionally, but not often, you'll find a [flagged] comment that isn't [dead], you can't vouch for those either. Only once they become [dead] can they be vouched for.