Tahini is a good substitute for nut butters. You can grind your own sesame seeds, if need be. Also, as others have mentioned, flax seeds are quite nutritious.
Vegan Pesach is quite a challenge, in terms of variety.
You are thinking of varenikas (not pierogi), also from Ukraine/Poland/Russia. I used to make them with sour cherries. They were larger than kreplach, shaped like a calzone, using a simple noodle dough, then boiled. Served with a sauce made from the same fruit as the filling.
I have made kreplach using seitan or Quorn Roast (not kosher-certified), because my son is allergic to soy. Any vegetarian chicken-substitute can be ground -up with sauteed onions and used as a filling. I cook them by boiling in broth, not water, so they have a little extra flavor.