Sources disagree about whether יי is pronounced [ej] or [aj] in "Ukrainish" (Southeastern) Yiddish, but no source I checked says that both pronunciations are used (in SEY that is). Note that I'm only talking about יי, not ייַ.
They were not totally consistent, but I believe this was because the community of Jews in Uruguay came from all over and they made up some kind of 'standard'. My big surprise was to realize (when I started studying Linguistics) that when I was a child, I had no problem talking and understanding my friend's parents' Yiddish but they were all using absolutely different vowels. Sounds funny but I haven't studied the issue at all. For 'child acquisition of language' it's an interesting problem.