Are two years acceptable to you? Notty will strip almost all the flavor out of juice if you let it go long enough. I've got two kegs of Notty/Johnathon which are almost 18 mo old and just now getting barely drinkable.
If you have the time and patience, those acids will break down eventually. If its a sour candy tangy you can use MLF to mellow that. A lot of times its a lot of citric acid and IFAIK that just takes time to break down
If the juice is sour or flat, but there is a decent apple taste in the finish, it's probably worth saving or backsweetening.
If its just a lot of acidic and mineralic flavors and no discernible apple, you're probably better off starting another batch than trying to salvage. When that happens, I give it to some friends for vinegar stock, or maybe see if you can freeze concentrate some taste back out of it.
With backsweetening you have a lot more options if you have kegs and/or can keep the resulting mix cold, so you dont have to worry about refermentation. You could also use k-meta followed by sorbate, although IMHO that just adds more bad tastes - although the vanilla bean might mask the sorbate taste. Either way I'd recommend taking out the vanilla bean. If the cider has picked up the vanilla bean taste, you might want to try cinnamon to balance it out
Sometimes different sweeteners work better than others for backsweeting, it depends on the acids left in the cider. Cane, corn, original juice, concentrate, honey & agave can all be good options, depending on what you end up with. I mostly use agave these days, but it depends on the juice as to what works best.
I havent had much luck with non-fermentables - lactose or Splenda, but some have had good results with those.
The problem with agave or any other fermentable is keeping it stable, because you'd need a lot of sugar to make this taste good anytime soon. If you can keep a keg cold in the fridge then problem solved, otherwise you can wait for a party or some other occasion where you can mix it, and it will all be consumed before re-fermentation could happen.
Next batch I'd advise skipping the nutrient and stop the fermentation before all the natural apple sugar is gone. The easiest way is with k-meta followed by sorbate, although IMHO cold crashing is worth the extra time and if you have kegs is easier. Pasteurizing can work if you have a suitable dishwasher that you can program to do a pasteurization temp cycle.
