Mother tongue vs vernacular

Question

Having been a non- native speaker, several times I had been asked to write my name IN VERNACULAR or IN MOTHER TONGUE during my college days. I couldn't tell apart the subtle difference and thought of them to be synonymous those days.

According to the Wikipedia article, VERNACULAR is the language spoken by a group of people inhabiting a particular region/ country which is regarded as of low - status.

Mother tongue is in some countries the language/ dialect of one's ethnic group rather than the first language — the one exposed to, from his birth.

Thus, to reach a conclusion,

1. The definitions of both terms overlap to a certain degree.

2. The definition of mother tongue seems little VAGUE.

Which out of the two is MORE REFINED & FORMAL to denote one's first language? I ask this on the grounds:-

1. The term vernacular has derogatory connotation.

2. On the contrary, I think that mother tongue is a SUBSET of vernacular and not vice versa. (In most cases, one's mother tongue is the product of their vernacular.)

Also, I'm personally of the opinion that if somebody asks you to use your vernacular, it indicates imperialistic approach of the instructor as witnessed by the excerpts given below. (I'm taken by the retro-noir-word "ANGLO-VERNACULAR").

Answer 1

Asking which of the two is more refined or more formal is misguided, because they have different meanings and operate within different ways of classifying languages/dialects.

Everybody has a mother tongue (i.e. a native language). The term mother tongue is neither informal nor particularly formal. (I suspect that some people could object to it on the ground that its use ignores those who were, in their early childhood, raised by somebody other than their mothers, but it doesn’t seem that this objection has had much influence on its use.)

The question ‘How do you write your name in your mother tongue?’ always has an answer. Somebody whose native language is Greek will respond to that question by using Greek characters, somebody whose native language is Japanese will respond by using Japanese characters, etc. The question ‘How do you write your name in the vernacular?’, on the other hand, may be confusing to many people. They may wonder ‘Which vernacular?’ Somebody whose native language is the standard contemporary Greek does not think of writing his name in Greek characters as using a vernacular.

I suspect that the OP has encountered such questions in a country in which there are many ethnic groups with distinct languages, but in which only a small number of languages are designated as official, and in which, moreover, most people learn one of the unofficial languages as their mother tongue. In such a setting, vernacular may be a standardly used term for the unofficial languages. Understanding the term that way, however, presupposes some familiarity with the difference between the status of different languages in that country, which an audience outside it may not possess (and which may not be important to them even if they do know about it). Because of that, mother tongue or native language are better terms than vernacular to use for this purpose when communicating with English-speaking audience of an unknown background.

(Incidentally, although it would be odd to ask in the present-day Europe, what somebody name in the vernacular is, such a question would have been perfectly apt a few centuries ago, when Latin was the language of formal communication in most of Western Europe.)