Vouch has been implemented for two CMSes so far in a way that is not very expensive for senders. Referrers are logged and processed to confirm they link to the site. If they do, they’re stored as potential vouch URLs. To select a vouch URL to send with a webmention, check the links on the target page against the domains of the potential vouch URLs. Validation by the receiver is pretty simple, as it should be.
I maintain the ProcessWire Webmention plugin that includes Vouch and run it on my own site. Vouch may not have wide adoption yet, but it has real applications and is not just “crap.”