In bowl, beat together cake mix, milk, vanilla,eggs. Add cream cheese and beat 2 minutes more until blended. Toss rhubarb in 1/4 cup flour. Stir into batter. Pour batter in greased 9x13 baking pan. In another bowl, make a crumble out of sugar, butter and the remainder 1/4 cup flour. Crumble over batter evenly. Bake in a preheated 325 degree oven for 40-45 minutes.
Note: If using frozen rhubarb, measure it out while it's still frozen THEN let it thaw. Drain in colander but don't squeeze out moisture.