Preheat your oven to 350°F (175°C).
In a mixing bowl, combine the chopped rhubarb and sugar. Let it sit while you prepare the batter.
In another bowl, mix the flour, baking soda, and salt.
In a separate bowl, combine the buttermilk, vanilla extract, and melted butter.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Fold in the rhubarb and sugar mixture.
Pour the batter into a greased baking pan.
Bake for 45 minutes or until a toothpick inserted into the center comes out clean.
Let the cake cool before serving.