Preheat oven to 350 degrees F (175 degrees C). Grease and flour one 9x13 inch pan.
In a medium bowl, stir together the flour, baking powder, baking soda and salt. Set aside.
In a large bowl, cream together the butter and sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla. Beat in the flour mixture alternately with the sour cream.
Fold in the rhubarb. Spread evenly into the prepared pan.
Bake for 40 to 45 minutes in the preheated oven, or until a toothpick inserted into the center of the cake comes out clean.