Preheat your oven to 375°F (190°C). Grease a muffin tin or line with paper liners.
In a large bowl, combine the bran cereal and almond milk. Let it sit for 5 minutes to soften.
In another bowl, whisk together the whole wheat flour, brown sugar, baking powder, baking soda, salt, and cinnamon.
Add the chopped rhubarb to the flour mixture and toss to coat.
Add the vegetable oil and vanilla extract to the bran mixture and stir to combine.
Gradually add the wet ingredients to the dry ingredients, stirring until just combined. Do not overmix.
Divide the batter evenly among the muffin cups.
Bake for 20 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 before transferring to a wire rack to cool completely.