Does anyone know when and by whom the Hasidic pronunciation started? I mean "burich" as opposed to "boruch". The same goes for Yiddish.. "vus" as opposed to "vos". I've heard that it was in opposition to Maskilim. Can anyone quote a rabbinic text? Is there a single person that introduced this?
This was not in opposition to Maskilim, and is not specific to Hasidim. In fact, dialectic variation in Yiddish (whence the Hebrew pronunciation) predates the Haskalah. Max Weinreich, on page 16 of the first volume of his געשיכטע פֿון דער ייִדישער שפּראַך (History of the Yiddish Language), says:
.1700 די הײַנטיקע ייִדישע דיאַלעקטן־סיסטעם האָט זיך אױסגעפֿורעמט צװישן 1500 און ...
... the present-day Yiddish dialect system formed between 1500 and 1700.
This is not to say that there were no phonetic differences in Yiddish spoken before this time. I'm far from an expert, but I imagine that he means that the major distinctions of Western and Eastern Yiddish, and perhaps major subdivisions with Eastern Yiddish, were distinguishable at that time, with their major characteristics recognisable.