My understanding is just that - it's the opposite of flag, however I thought there was some karma threshold or something that permitted vouch to occur, where as flag was fairly immediate. Maybe is just needs to be dead first.
Today’s new feature lets users rescue [dead] posts on a
case by case basis. Beside the ‘flag’ link, you’ll see a
‘vouch’ link to click when a post should not be [dead].
When enough users vouch for a post, the software will
unkill it. Think of vouches as the inverse of flags: a
flag says that a post shouldn’t be on HN; a vouch says it