Vouch buttons show up when a post is [dead], not when it's [flagged]. I unkilled that comment a while ago*, so it's no longer [dead], so there's no longer a vouch button.
* normally we wouldn't do that, but in threads that have a YC connection we moderate less, not more - see https://hn.algolia.com/?dateRange=all&page=0&prefix=false&qu...