intransitive verb
to affirm or certify someone or something to be true, truthful, genuine, or correct (usu. fol. by “for”).
The witness vouched for the defendant’s good character.
I’m willing to vouch for him because I’ve never known him to lie.
transitive verb
to confirm or substantiate by providing evidence, as with practical proof or demonstration.
Can you vouch your whereabouts at the time of the murder?