Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners.
In a large bowl, mix together the flour, sugar, baking powder, and salt.
In another bowl, whisk together the milk, oil, beaten egg, and vanilla extract.
Add the wet ingredients to the dry ingredients and stir until just combined.
Fold in the chopped rhubarb.
Spoon the batter into the prepared muffin tin, filling each cup about two-thirds full.
Bake for 20-25 minutes or until a toothpick inserted into the center of a muffin comes out clean.
Allow the muffins to cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.