Why is my lcd randomly touched by itself

The issue you're seeing is commonly known as "ghost touch" and is caused by a damaged connection between the digitizer screen and the flex cable leading to the motherboard. That bad connection allows electrical noise onto the signal line that gets picked up by the processor and interpreted as a touch input. Unfortunately, that kind of damage isn't repairable and the only fix is to replace the screen.

I wasn't able to locate any iFixit-style guides for you, but the device page has a link to a YouTube video showing the process.

Vivo S1 Display Replacement - Broken Screen change - YouTube