Vouch is a proposal for abuse mitigation.
If A is sending a webmention to B (who don't know each other, i.e. B has never linked to A), A sends along a third parameter, "vouch=C", where C is a URL to a page that has a link to A's domain (thus "vouching" for A), and C's site is a domain that B approves of (e.g. has linked to in the past).
By doing so, A is telling B: please accept my webmention, because C, who you know or approve of, vouches for me.