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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The Collaborative International Dictionary of English v.0.48:

Vouch \Vouch\, v. i.
   1. To bear witness; to give testimony or full attestation.
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            He will not believe her until the elector of Hanover
            shall vouch for the truth of what she has . . .
            affirmed.                             --Swift.
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   2. To assert; to aver; to declare. --Shak.
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The Collaborative International Dictionary of English v.0.48:

Vouch \Vouch\, n.
   Warrant; attestation. [Obs.]
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         The vouch of very malice itself.         --Shak.
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WordNet (r) 3.0 (2006):

vouch
    v 1: give personal assurance; guarantee; "Will he vouch for me?"
    2: give surety or assume responsibility; "I vouch for the
       quality of my products" [syn: guarantee, vouch]
    3: summon (a vouchee) into court to warrant or defend a title
    4: give supporting evidence; "He vouched his words by his deeds"