GP was banned years and years ago for something (I don't know what) and now their comments are all [dead] on arrival. People (sibling commenter to you did this) can vouch for these [dead] comments ([banned] cannot be vouched, IIRC, but is very rare). Also upvoting and vouching are separate things here. You vouch [dead] or [flagged][dead] comments, you can only upvote live comments. You never need to upvote before replying to a comment.