Place the blueberries in a small bowl and mix with vanilla and lemon juice. Sprinkle with ½ cup of sugar and 1 tsp of flour, then stir in the 1½ tsp of melted butter. Spoon mixture into the base of the loaf pan.
In a medium bowl, stir together 1 cup of flour, baking powder, salt, and 3 Tbsp sugar.
Add 2½ Tbsp butter and, using either your fingers, a fork, or a pastry blender, combine until the butter is dispersed throughout the flour in small pieces.
Add milk to a measuring cup and add egg; beat the mixture until thoroughly combined.
Make a well in the center of the butter/flour mixture and stir in milk and egg mixture. Combine just until moistened, forming a thick batter. Cover with a clean towel and let batter rest for 10 minutes.
Spoon the batter over the blueberries. Mix together the cinnamon and sugar; sprinkle over the top.
Bake for 25 minutes or until the top is starting to brown. A knife inserted into the topping should come out clean. Let cool before serving. Serve with whip topping.