Preheat Oven to 400 degrees F.
Crust: Combine dry ingredients, cut in chilled butter cubes until mixture resembles coarse crumbs. Add egg, water and vinegar, handle as little as possible. Add more water if needed, until a handful of the dough mixture holds together in the palm of your hand when squeezed. Pat the dough into a ball gently. Cut in 1/2. Wrap in plastic wrap, then pat into disks. Refrigerate while making the filling. (*Freeze if you are a fast apple peeler!) *When you are ready to roll out, remove crust from refrigerator or freezer, roll out to a 13 inch round. Place one round in bottom of pie plate, do not stretch. Cut strips out of the other, and basket weave over the top of pie in lattice fashion.
Filling: Core and peel apples. Slice to 1/4" thickness. Add zest of lemon and juice. Add sugar and rub zest together with sugar to release oils of lemon into the sugar. Add flour, vanilla and a pinch of salt. Stir well until incorporated, using care not to break up the apple slices. Pour into pie plate that is lined with one crust, finish top crust with lattice style strips or one flat pastry top with vents cut out. Roll crust over top, crimp to seal the seams.
Egg Wash for Crust Topping: In a small bowl, combine egg and water, whisk well. Brush over prepared pie. Sprinkle with sugar. Place a foil collar (aluminum foil, folded in thirds lengthwise and wrapped around pie) or pie crust shield around edge of pie for baking. Remove foil collar for last 10-15 minutes of baking.
Bake: in a preheated 400 degree oven for 20 minutes, then reduce heat to 350 and bake for 40-50 minutes until slow bubbles form in the juices around the edges of pie (you can see this through the lattice crust or vent holes). If the bubbles are quickly popping, the filling is not thick enough yet. Cover with foil if center of pie is overbrowning towards the end of baking (*see photo on my post, I need to follow my own advice). Remove foil collar for end of baking to brown crust as desired. Remove from oven, let cool for 4 hours so pie will 'set' properly. (*won't be runny.) Makes one 9" pie.