Pre-heat oven to 400 degrees (don't forget to put a drip pan under your pie).
Peel, core and slice apples and soak in cold water with the lemon juice (this will keep them from turning brown).
Combine sugar, flour, cinnamon, salt together, then add vinegar. Mix well.
Drain blueberries and apples then mix together. Mix with the flavoring until well coated.
Put one rolled out pie crust on the bottom of your pie pan. *Bottom crust can be pre-baked slightly to prevent soggy bottom (no body likes that).
Poke some holes in the crust with a fork and bake at 400 degrees for 5 minutes.
Add apples, spreading out evenly (there will be extra sugar/flavor coating, drizzle some over the apples).
Cut butter into cubes and put on filling.
Place the top pie crust on the top of apples, the edges will be difficult to crimp with the top crust if you pre-baked the bottom crust so you may want to fold the top crust under and then crimp it.
Cut 4 small vent holes on top.
Crimp edges of pie crust together.
Cover edges of pie with foil to protect them from burning, then add an additional foil over the whole pie.
Bake at 400 degrees for 45 minutes, then take off top foil, leaving edges covered.
Baste top of pie with mixture of egg yolk and cream (this will give you a really flaky shiny crust) bake for another 10 minutes then take off remaining foil and baste edges with egg yolk/cream mixture.
Bake for another 5 minutes or until pie golden.
*for plain apple pie omit blueberries.
**You can also substitute dried cranberries for blueberries for a delicious apple-cranberry pie.