From The Collaborative International Dictionary of English v.0.48:
Vouch \Vouch\, v. t. [imp. & p. p. Vouched; p. pr. & vb. n.
Vouching.] [OE. vouchen, OF. vochier to call, fr. L. vocare
to call, fr. vox, vocis, voice. See Voice, and cf.
Avouch.]
[1913 Webster]
1. To call; to summon. [Obs.]
[1913 Webster]
[They] vouch (as I might say) to their aid the
authority of the writers. --Sir T.
Elyot.
[1913 Webster]
2. To call upon to witness; to obtest.
[1913 Webster]
Vouch the silent stars and conscious moon. --Dryden.
[1913 Webster]
3. To warrant; to maintain by affirmations; to attest; to
affirm; to avouch.
[1913 Webster]
They made him ashamed to vouch the truth of the
relation, and afterwards to credit it. --Atterbury.
[1913 Webster]
4. To back; to support; to confirm; to establish.
[1913 Webster]
Me damp horror chilled
At such bold words vouched with a deed so bold.
--Milton.
[1913 Webster]
5. (Law) To call into court to warrant and defend, or to make
good a warranty of title.
[1913 Webster]
He vouches the tenant in tail, who vouches over the
common vouchee. --Blackstone.
[1913 Webster]
Syn: To obtest; declare; affirm; attest; warrant; confirm;
asseverate; aver; protest; assure.
[1913 Webster]