What is the difference between a vernacular and a dialect ?
The vernacular is the language spoken by the ordinary man [ie, non-academic]
Dialect is the form of a language which is peculiar to a specific region or social group.
There is a subtle difference, in that [for instance] the poet Robert Burns wrote his poems in the Scots vernacular, whereas D.H Lawrence wrote poems in Lancashire dialect.
According to Webster online these are very similar terms. But in looking at the definitions I think that a vernacular can be whole separate language while a dialect is just a modification of a language. Perhaps you will get a better answer from someone else.
vernacular "1 a: using a language or dialect native to a region or country rather than a literary, cultured, or foreign language b: of, relating to, or being a nonstandard language or dialect of a place, region, or country c: of, relating to, or being the normal spoken form of a language"
dialect "1 a: a regional variety of language distinguished by features of vocabulary, grammar, and pronunciation from other regional varieties and constituting together with them a single language <the Doric dialect of ancient Greek> b: one of two or more cognate languages <French and Italian are Romance dialects> c: a variety of a language used by the members of a group <such dialects as politics and advertising — Philip Howard> d: a variety of language whose identity is fixed by a factor other than geography (as social class) <spoke a rough peasant dialect>"