Dang, this shouldn't be flagged. This is hugely important and relevant.
Where is the mythical vouch feature I could never find ? A yes I believe it's not yet dead, so you can't vouch ?
With a comment, I think that if it's flagged (more than some number of times), then it's dead. (It's also dead if the user is shadowbanned, or if it's downvoted to -4.) If you think that's wrong, you can (and should!) vouch for it.
With a post, it can be flagged but not dead. Such threads can be replied to. Or a thread can be dead, in which case it cannot be replied to, but it can be vouched.
That's my understanding, but I could be wrong.