For the filling: Melt the butter in a large skillet over medium-high heat. Add the apples and vanilla bean and saute for about 2 minutes. Add the sugar, cinnamon and salt to the pan and cook until the apples start to turn a light caramel-golden brown but aren't falling apart, 6 to 8 minutes. The apples should still have some tooth to them. Add the flour and cook for an additional 3 to 5 minutes, to remove the raw taste from the flour. Fold in the lemon juice. Spread the filling out in an even layer on a baking sheet and refrigerate until cold.
For the egg rolls: Arrange an egg roll wrapper in a diamond shape on a board in front of you. Brush the borders with beaten egg. Spoon 2 to 3 heaping tablespoons of the filling into the center of the wrapper. Take care to leave about 1 inch between the filling and the edges of the wrapper. Fold the end of the wrapper nearest you over the filling, then fold the sides over. Finally, roll the wrapper into a tight cylinder, tucking and tightening as you go.
Repeat with the remaining wrappers until all the filling is used. As you go, remember to keep the unused wrappers, as well as the finished egg rolls, covered with a towel to prevent them from drying out.
Add 4 to 5 inches of oil to a heavy-bottomed pot or Dutch oven and heat to 350 degrees F.
Fry the egg rolls in batches of 4 until golden brown, 8 to 10 minutes. Transfer to a paper-towel-lined baking sheet.
Serve dusted with powdered sugar.