So I chose to friendly Death’s Vigil on Normal-Veteran, and to friendly Kymon’s Chosen in Elite, will I be able to friendly Vigil in ulti, and which one will the game chose for my friendly ? It will change every time I change diff levels ?
The game will not choose for you.
You’ll be friendly to the faction that you choose in each difficulty.
Does this work for Anastheria too? And Barrowholm?
Well i killed Anasteria on normal/veteran and on elite i met with her and said “i want to make thing straight” and u can friendly here again - borrowholm dont know yet, just started last act AoM on elite !
I killed them at 1st sight on normal/veteran, but ill see very soon if u can friendly them after that.
Cool. I did the same with my Cabalist, since I figured this is an easy way to finally get to Honored with the Black Legion, and I could not use her augments anyway until I hit level 70, so that can wait until Elite.
Ultimately, what you chose in Ultimate is all that matters. After that choice, your faction reputation will change for all 3 difficulties.
Just to complete that, what matters is your most recent choice no your Ultimate choice. You can very well not choose in Normal and Elite 
Except that on Normal and Elite, you can always make a change once you hit Ultimate. After Ultimate, you are locked in.
Hmm, thanks! Did not know. But that makes sense.
I was playing ultimate at 85 going into the expansion for my first lvl 100. I didn’t remake one of the new classes. I chose not to side with barrow in the beginning in ultimate. This got me hostile with everything. I later played Elite and was able to change my decision which affects my standings in Ultimate so that should be taken into account. You’re not locked in Ultimate if it’s not your last decision as previously mentioned.
I love the kill first approach. 