1. Heat oven to 400 degrees Fahrenheit.
2. To make the dough: In a food processor, mix flour, sugar, and salt. Using on and off pulses, blend margarine into flour mixture, forming pea-size crumbles. In a small bowl, whisk non-dairy milk and egg in a separate bowl. Pulse into flour mixture. Blend dough until smooth but with visible flecks of margarine. Do not overmix. Flatten dough into a disk and wrap in plastic wrap. Chill at least one hour before using.
3. To make the filling: Melt margarine in a two-quart saucepan over medium heat. Add strawberries and rhubarb and cook until soft and jamlike, six to eight minutes. Stir in sugar, vinegar, lemon juice, and salt; cook three minutes more. Remove from heat and add vanilla extract. Let cool to room temperature, then cover and chill at least 30 minutes before using.
4. To assemble and bake the pies: On a lightly floured surface, roll dough to 1⁄4 inch thick. Trim edges to make a 10×12 inch rectangle. Cut dough into six three inch by four inch rectangles; gather dough scraps, re-roll, and cut one more rectangle. Place two tablespoons filling in the center of each rectangle. Brush edges of rectangles with egg wash. Fold one short side of dough over the other, encasing the filling. Crimp edges using a fork. Transfer pies to parchment paper-lined baking sheets and chill 20 minutes.
5. Using a sharp knife, make a few slits on the top of each hand pie. Brush with remaining egg wash. Bake pies until golden, about 17-20 minutes; let cool slightly before serving.