Webster’s 1913 has no entry under this spelling.
Shades of meaning
Fernald has no entry of its own for mother tongue; he lists it under language.
Where he writes about it:
Vernacular, from the Latin, has the same general sense as the Saxon mother tongue, of one's native language, or that of a people; as, the Scriptures were translated into the vernacular. — from language
Antonyms
none given