I wouldn't suggest raising the FC to 20 ppm with a vinyl liner pool. By chemgeeks calculations each pound of dichlor will raise FC about 7 ppm rounded off) and CYA about 6 ppm. I would raise the FC like watermom suggested with dichlor until 4 lbs had been used in total and then switch to bleach to maintain 12-15 ppm until the water is clear.