The Collaborative International Dictionary of English v.0.48:

Vernacular \Ver*nac"u*lar\, a. [L. vernaculus born in one's
   house, native, fr. verna a slave born in his master's house,
   a native, probably akin to Skr. vas to dwell, E. was.]
   Belonging to the country of one's birth; one's own by birth
   or nature; native; indigenous; -- now used chiefly of
   language; as, English is our vernacular language. "A
   vernacular disease." --Harvey.
   [1913 Webster]

         His skill in the vernacular dialect of the Celtic
         tongue.                                  --Fuller.
   [1913 Webster]

         Which in our vernacular idiom may be thus interpreted.
                                                  --Pope.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Vernacular \Ver*nac"u*lar\, n.
   The vernacular language; one's mother tongue; often, the
   common forms of expression in a particular locality, opposed
   to literary or learned forms.
   [1913 Webster + PJC]

WordNet (r) 3.0 (2006):

vernacular
    adj 1: being or characteristic of or appropriate to everyday
           language; "common parlance"; "a vernacular term";
           "vernacular speakers"; "the vulgar tongue of the masses";
           "the technical and vulgar names for an animal species"
           [syn: common, vernacular, vulgar]
    n 1: a characteristic language of a particular group (as among
         thieves); "they don't speak our lingo" [syn: slang,
         cant, jargon, lingo, argot, patois, vernacular]
    2: the everyday speech of the people (as distinguished from
       literary language)