You don't normally want to ever drain and refill an in-ground vinyl pool, and even if you could it is very likely to be simpler to get the PH under control and then shock the pool to kill the algae.
First, get the PH down to 7.2 with acid. Then shock the pool. You should use bleach to bring the FC level up to shock level, and can then use the SWG to help maintain shock level, though it will probably still need bleach to supplement the SWG.
First, get the PH down to 7.2 with acid. Then shock the pool. You should use bleach to bring the FC level up to shock level, and can then use the SWG to help maintain shock level, though it will probably still need bleach to supplement the SWG.