If you leave water in a vase too long it can stain. The easiest way to clean this is to pour white vinegar into the vase above the stained area. Let it sit for a few hours. Next, add a teaspoon of uncooked rice. Cover the top of the vase with your hand or something else and shake. The vinegar will have helped rid the stain while the grains of rice will scrub away any residue. Pour everything out and rinse.