Preheat your oven to 350°F. Grease an 8 x 4-inch loaf pan.
In a large bowl, whisk together the flour, baking soda, salt, and cinnamon. Stir in the brown sugar, oil, buttermilk, egg, and vanilla until everything is well combined. Gently fold in the diced rhubarb and chopped pecans.
Pour the batter into the greased loaf pan, spreading it evenly. Bake for about 50 to 55 minutes, or until a toothpick inserted into the center comes out clean. Allow the bread to cool in the pan for a bit, then transfer it to a rack to cool completely before slicing.