Preheat your oven to 400°F (200°C). Grease a muffin tin or line with paper liners.
In a large bowl, whisk together the flour, sugar, baking powder, baking soda, and salt.
In another bowl, combine the buttermilk, oil, vanilla extract, and egg. Mix well.
Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix.
Fold in the chopped rhubarb.
Divide the batter evenly among the muffin cups.
Bake for 20-25 minutes, or until a toothpick inserted into the center comes out clean.
Let the muffins cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.