I know of no substitute for the vanillin sugar, you might want to check gourmet shops or specialty shops. Possibly a custard mix might work.
Instructions:1. Roll out the pastry to a rectangle 14x18 in and brush with the fat.
2. For the Filling: mix together the filling ingredients and distribute evenly over the pastry.
3. Starting from the shorter side, roll up like a swiss roll.
4. Then use a sharp knife to cut off slices about 1 1/4 in thick.
5. Lay these on a greased baking sheet and flatten slightly.
6. Preheat oven for 5 min at very hot (400F), then bake for 15-20 minutes.
7. For the icing: sieve the icing sugar (powdered sugar) and blend with as much of the water as will five a good coating consistency.
8. Ice the schnecken while still hot.