I'm not sure. Did you try some ph down? Since your pool is plaster, what about using something like a "stain eraser" or a pumice stone. You can use the stain eraser - it is sold at most pool stores - or online. It is like a rubber eraser with a little pumice in it. I'm just thinking that you need something to rub the vitamin c or ph down into the stain a little harder. You can also put in some sequestering agent, run your ph between 7 & 7.2, and see if that lifts the stain off. Let us know the results.