Several people have implemented receiving vouches. It is relatively easy to look at a vouch URL and see if it links to a third-party who you have approved of in the past. While there are more advanced things you can do, that is the basic summary of the protocol.
The harder part, and less implemented by others is sending of vouches. Where do you find people who have been approved by people you have approved of? It would really help if we had some more discussion on this.