When my wife was still alive we always had a king size bed. Always used a plastic/vinyl cover directly over the mattress but then a more comfortable topper over that. The problem was the diapers I wore at night had more leakage, sometimes severe. The topper was more difficult to wash and dry, sometimes they needed washed several days in a row. The topper wouldn't fit in the same load as the rest of the sheets. Then tried a pad over sheets but would always bunch up or get lost through the night. Then put the pad under the sheets but on top of the topper. This mostly worked and only had to wash the sheets if there was leakage. Sometimes the leakage missed the pad got on topper anyway and back to washing everything in the morning. My wife I both worked so sticking around to do the laundry was a major pain. After she passed I went back to just using the plastic/vinyl mattress cover under the sheets. The sheets fit in one load so I could throw them in the wash and dry them when I got home. Used disinfecting cleaner on the plastic cover and let it dry and air out all day. Sleeping on a plastic mattress cover was what I slept on all my life up until I got married. The vinyl style nowadays is more quiet then the plastic ones from my childhood. The comfy topper was for her comfort which I was glad to do for her. A couple years ago I switched to ConfyDry 24/7 diapers and started using a booster. I no longer have problems with leaks at night but continue to just use the vinyl mattress cover anyways. It is easier to keep an extra set or two of sheets than it is extra mattress toppers. Don't know if this helped, it's just been my experience.
Al.