FYI, vouching someone isn't any skill or ability. You vouch when you are telling everyone that you are 100% sure that the person you vouch isn't yandere.
You can vouch someone (perhaps civilian) whom you want to be killed by yandere. Yandere like to kill vouched people, and for a reason!
When you can not guarantee that someone isn't yandere tell clearly that you half-vouch or 90% vouch for someone.
When there are more than 1 yandere, make sure that those yanderes aren't fake vouching each other.
See also: http://en.wiktionary.org/wiki/vouch