To certify, confirm that something or someone is bona fide or qualified. To call upon; to call in to warranty ; to call upon the grantor or warrantor to defend the title to an estate. To vouch is to call upon, rely on, or quote as an authority. Thus, in the old writers, to vouch a case or report is to quote it as an authority.
VOUCH
TheLaw.com Law Dictionary & Black's Law Dictionary 2nd Ed.