Stir graham crumbs, sugar, and melted butter in a bowl until evenly moistened.
Press the mixture firmly into a 9-inch pie plate, going up the sides. Chill 15 minutes.
Beat cold heavy cream with powdered sugar and vanilla to soft peaks; set aside.
Whisk instant pudding mix with cold milk for 2 minutes until thick and smooth.
Fold about half of the whipped cream into the pudding to make it light and airy.
Slice bananas and toss with lemon juice if using. Layer slices over the chilled crust.
Spread the creamy pudding mixture over the bananas and smooth the top.
Swirl the remaining whipped cream over the pie for a fluffy finish.
Chill 2–4 hours, until set. Slice and serve cold.