How do you get red hair dye off vinyl floor?

If it is a small area, scrub it with a scouring pad. If it is a large area, consider replacing the vinyl.

For two reasons:

Any harsh chemical that you can use to remove the dye will corrode the varnish coating on the vinyl. That layer that makes the vinyl easy to clean. It will in other Words, ruin the vinyl.

But the worst thing is... hair dyes usually have ammonia in them. And mixing ammonia with some household cleaners, specially those containing bleach, is extremely dangerous. The chemical reaction causes a poisonous gas to be released in the air. This gas is so dangerous it was used in warfare during WWI.

If you dye your hair frequently, consider using a big garbage bag to line the floor so that the dye will not drip on the vinyl.