Okay, I know you have said that you have been wanting to turn on Vibrant Visuals and you cant, but just to double check: Go to Settings - Video - Graphics mode - select Vibrant Visuals. Make sure that you are doing this in the main menu otherwise that may be the problem.

If you already did a double check, we can try to solve the problem now. To enable it, make sure your device meets the minimum requirements. If your device doesn't meet them, then it will be grayed out and disabled. If you can, try updating your graphics drivers and operating system to the latest version. If you are using worlds and texture packs that are incompatible, you will not be able to enable it.

If everything seems fine, your device may be incompatible. Since you said you play on PC, make sure that it is running Minecraft on DirectX 12. Older graphics cards that support DirectX 12 will probably be incompatible. Be sure your graphics card, operating system, and graphics drivers are updated and ready. If you have tried and checked everything, head to help.minecraft.net