I am a glass wearer (around -2 dioptres, myopic vision). When you are shortsighted, you normally wear your glasses. If you then put dioptres on the viewfinder, you are supposed, to avoid strain, to take your glasses away, and put them again, any time you bring the camera to the eyes.
Besides contact lenses, the solution is to just keep the glasses on. I can see information on the viewfinder, if needed I just tilt very slightly my eye on one side. I basically see all.
So my advice for short-sighted people is to buy some rubber washer for the viewfinder ocular in case there isn't any, and just keep your glasses as usual. After a while you get used to it.
If I get it right, my lens + camera cannot serve as a dioptre, so there is no risk, in principle, that I focus on the wrong plane. But if you don't see clearly the viewfinder image the difference between in-focus and out-of-focus is harder to detect.