@ filip: This is exactly the romanian language, written almost the same and pronounced exactly the same. Ofcouse, the language has also been highly influenced by the serbian language, borrowing some serbian words (comcie, drugar, etc).

When it comes down to what serbian people call "vlachs" there are two types of people: aromanians and romanians.

Aromanian people have always been in danger of being assimilated, being spread around so many other nations, so they placed a lot of importance to their own language and national identity, not wanting to consider themselfs tied to any nation. So yeah, they have their own orthography and rules, but not so strict as other people. They mostly use the spoken language (a word it's written as it sounds) so most of them are able to comunicate very easily with eachother. Now, this language which has a lot of balkan influence to it is called aromanian language, but it still has it's own core that it's not borrowed from other languages and which is very similar with the romanian language. It is a known fact that in the old times, aromanian and romanian people were speaking 2 languages that were even more closely related than todays romanian and aromanian languages. And that is amazing, considering that even today those two languages are pretty close.

The romanians from Serbia always called themselfs "rumuni", as you can see in their own popular and ethnic songs: https://lyricstranslate.com/en/la-no%C4%AD-rum%C3%A2%C5%84-d%C3%AEn-s%C3...
Only in recent times there has been a high pressure of them to reject their ethnic inheritance and be some no name no nation "vlachs". This is mostly because of the bad job done by the Romanian External Affairs Office but also because of a social shuning caused by the serbs and some serbian patriotics among the romanian ethnics. They mostly promote the idea of "if you're vlach you're with us, if your rumuni you're against us".

The idea of preserving and promoting their language and identity is a noble one by itself, something worth of prise. But doing so by rejecting you own origin is wrong, it goes against everything that you are. Our ancestors never called themselfs vlachs, be them romanians or aromanians, they always called themselfs rumun, arman, raman, remen etc. In fact, the only ones that called us vlachs were either enemies or complete stragers. So when we call ourselfs vlachs, if not enemies we are at least strangers to our own people.