OK, let's clear up a misconception:
You CAN dump half the water in your vinyl pool safely. You just need to make sure there's at least 1 foot of water in the shallowest area, then refill. You don't need the Poconos Plastic Sheet method for that.

A CYA of 50ppm is the high end of normal, but still normal--and a good number for a VERY hot,sunny area and lots of usage. That's where you get to if your CYA is 100 and you replace half your water.

Hope that helps.