My CYA is high (100 ppm), calculator tells me to replace 50% of the water. How much can I safely drain? Do I need to use a vacuum when the water level is below the skimmer? Can the water be replaced a little at a time?

in ground 32x16

You will need something once the water gets below the skimmer. It just depends on your setup. If the yard is higher than the street, you can use a garden hose, maybe even a pool vacuum hose and siphon. Or buy or rent a submersible pump.

You can do it in stages. If the only problem you have is high CYA - otherwise clear and balanced - that's workable. I've done it. It's not as efficient, though. Let's say you can drain 3" of water each time. And I'm assuming, perhaps incorrectly, that your pool is 32" deep. So, you lose 10%. If you are exactly at 100, the first drain brings CYA down to 90. The next drain brings you to 81. Then 73. Then 66, 59, 53, 48. And then you stop. 21" drained out of 32"

The other thing to consider is, are you really at 100? I'll quote myself from another recent thread: "Let's say your CYA is 100. It will read as 100. If your CYA is 120, it will still read as 100. But if you mix pool water with tap water (tap water has no CYA in it) 50/50, then use that for the test, you will read half, more-or-less, the actual value. So, CYA 100 will read as 50; CYA 120 will read as 60. Since you halved the sample, double the result.

What you're actually pouring into the test tube is 25% pool water, 25% tap water, and 50% R-0013 reagent. "