Preheat the oven to 350°F (175°C) and line a muffin tin with paper liners or grease it lightly.
In a large bowl, combine the mashed bananas, vegetable oil, sugar, egg, and vanilla extract. Mix until well combined.
In a separate bowl, whisk together the baking soda, salt, and flour.
Gradually add the dry ingredients to the banana mixture, stirring until just combined. Do not overmix.
Gently fold in the raspberries, being careful not to break them apart.
Divide the batter evenly among the muffin cups, filling each about 2/3 full.
Bake for 18-20 minutes or until a toothpick inserted into the center comes out clean.
Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.