Could be trivial but would like to know, what is the difference between
déjà vu and
Is it primarily the pronunciation difference or something else?
You should always spell phrases from other languages correctly if you can. In the past it was sometimes difficult to find the correct symbol on your keyboard or printing device. For example it was impossible to write déjà vu on a standard English-language typewriter.
The accents make a difference to the pronunciation using either English or French rules. In this particular example they do not help you much if you cannot read French but at least they remind you not to try to read it as English.
It makes a much bigger difference with e at the end of a word as this is silent, in both English and French, so cafe has one syllable but café two. There are complications though as some words have entered the English language without accents. Cafe and café both exist but often with different meanings and pronunciations.
Things are further complicated by different rules sometimes being applied. The BBC was notable for deliberately misspelling words by leaving out all accents on their website, so even people's names were misspelt such as *Beyonce but I am pleased to see that this rule is being quietly dropped.