What is Vouch?

Legal Definition
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 ls to quote it as an authority. Co. Litt. 70a.
-- Black's Law Dictionary
Legal Definition
To testify to having signed an instrument as an attesting witness. See 1 Root (Conn.), 224. To request a warrantor to defend the title.
-- Ballentine's Law Dictionary