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"

Moby Thesaurus II by Grady Ward, 1.0:

84 Moby Thesaurus words for "vouch":
   acknowledge, affirm, affirmance, affirmation, allegation, allege,
   and candle, announcement, annunciation, assert under oath,
   assertion, asseverate, asseveration, assure, attest, aver,
   averment, avouch, avouchment, avow, avowal, back up, bear witness,
   book, certify, conclusion, confirm, corroborate, countersign,
   creed, declaration, depone, depose, dictum, disclose, endorse,
   enunciation, give evidence, guarantee, ipse dixit, kiss the book,
   make a promise, manifesto, pledge, plight, position,
   position paper, positive declaration, predicate, predication,
   proclamation, profession, promise, pronouncement, proposition,
   protest, protestation, prove, say, say-so, saying, sponsor, stance,
   stand, statement, substantiate, support, swear, swear by bell,
   swear the truth, swear to, swear to God, swear to goodness,
   testify, troth, underwrite, uphold, utterance, verify, vouch for,
   vow, warrant, witness, word