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.]
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   1. To call; to summon. [Obs.]
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            [They] vouch (as I might say) to their aid the
            authority of the writers.             --Sir T.
                                                  Elyot.
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   2. To call upon to witness; to obtest.
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            Vouch the silent stars and conscious moon. --Dryden.
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   3. To warrant; to maintain by affirmations; to attest; to
      affirm; to avouch.
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            They made him ashamed to vouch the truth of the
            relation, and afterwards to credit it. --Atterbury.
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   4. To back; to support; to confirm; to establish.
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            Me damp horror chilled
            At such bold words vouched with a deed so bold.
                                                  --Milton.
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   5. (Law) To call into court to warrant and defend, or to make
      good a warranty of title.
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            He vouches the tenant in tail, who vouches over the
            common vouchee.                       --Blackstone.
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   Syn: To obtest; declare; affirm; attest; warrant; confirm;
        asseverate; aver; protest; assure.
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