For the crust
Whisk flours in a large bowl.
Cut cold shortening into flour using hands, pastry cutter or two knives until it is in pea-sized bits.
Using a fork stir in gently the vinegar and water 1 Tbs at a time until the dough comes together. You don't want it to be too wet. Form dough into a ball and flatten into a disk.
On a well-floured surface, roll out dough to about a 1/4" thick circle. Carefully drape dough over rolling pin and transfer to a pie plate or baking vessel of your choice and let dough hang over the edges (should be about 1-2").
For the filling
Peel and cut rhubarb into 1/2" slices.
Toss everything in a large bowl until evenly incorporated.
Spoon filling into pie dough. Fold overhanging dough up over the filling overlapping as needed.
Brush dough with soymilk (nut milk works) and sprinkle with rapadura sugar (or sugar of your choice).
Place pie on a foil-lined baking sheet (to collect juice drippings should they occur). Place in a 425-degree F. preheated oven and bake for 30 minutes.
Lower temp to 375 and bake for 20-30 minutes more. Juices should be bubbling and crust should be golden brown.
Let cool on rack for 1/2 hour.
Serve with vanilla ice cream. Enjoy!