Preheat the oven to 375°F with the rack placed in the center of the oven. Line a muffin pan with cupcake liners and set aside.
Make the Streusel:
In a small bowl, stir together the sugar, flour and cinnamon with a fork. Drop in the butter, and using your fingers, a pastry cutter or two forks, cut the butter into the sugar mixture until you have crumbs. Set aside.
Make the Muffin Batter:
In a medium bowl, whisk together the flour, sugar, baking powder and salt until combined. In a small bowl, whisk together the oil, egg, buttermilk and vanilla. Add the buttermilk mixture into the flour and use a spatula to mix until just about halfway incorporated. Drop in the blueberries and mix a little bit more, just until the flour is incorporated – do not overmix the batter, you want it to be lumpy.
Distribute the batter into the prepared muffin pan and sprinkle with the streusel. Place in the oven and bake for about 18-20 minutes, or until a toothpick inserted into the center comes out clean. Remove from oven and place the muffins on a wire rack to cool before enjoying.