only way is to drain and refill. you have a sand filter I see, so you can just adjust your pump to only suck from the main drain (bottom) so that you are not drawing water from the skimmer. then just set your filter to vacuum to waste. that will discharge the water. draw it down to the desired level and then refill with water from the hose.
its always more efficient to do larger drain and refill than multiple smaller ones. do you have a high water table? with vinyl, you don't want to go any lower than a foot or so above your shallow end. so if your shallow end is 3 feet deep, you can take 2 feet of water out.
with CYA of 90 and aiming for say 50, that means you have to take 45% of the water out. what is your average pool depth? if you shallow is 3 feet and your deep is say 6 feet, and they are roughly the same area, your average depth is 4.5 feet, so 45% would 2 feet of water. just need to calculate your average depth and then go from there to determine how low the water needs to go.
in the same example, if you had a 3 foot shallow end, a 6 foot deep end, but the deep end was 1/3 of your pool area...the average depth of your pool would be 3.5 feet, and 45% would be 1.58 feet.
all that said, its already basically August. how much longer will your pool be open? might be easier to just keep it where its at this year and then deal with it next year. besides, with cold weather a lot of people lose CYA over the winter, so you might get lucky. maybe just drain and refill 1 foot now and worry about it later.
