For recent dead comments, click on the time ("31 minutes ago") to bring up the comment's own page where there's a "vouch" option next to the other comment optio

For recent dead comments, click on the time ("31 minutes ago") to bring up the comment's own page where there's a "vouch" option next to the other comment options if you have enough karma. For dead stories in the "new" queue, I see a "vouch" option already without going to the story's own page.