I believe a red coder will outdo a gray coder even if the former uses notepad. Do you get the idea?

is Vim good for competitive coding? Yes if you master vim you can do any text typing/editing faster including coding.
Should you learn it? it's not a necessary for competitive programming you can use any text editor and you will be fine. But if you want to learn vim to make you a better developer Go and learn it, otherwise you can use any text editor:(Sublim, Vscode, atom) or use a vim plug-gin in your IDE like I did with xcode and now I'm using MacVim.

Learning vim wouldn't have any direct or significant effect on your CP skills. But if you are into other stuff as well like development, vim is a great tool to learn. If CP skills are your primary concern, practising more should be your priority imo.

I don't know about others but in my opinion one should. Touch typing and learning vim really helps.I started learning vim specifically for competitive programming about few weeks ago and it really helps a lot. In CP you constantly need to change lines, copy pasting and editing is there and in like first few problems even 2 mins can change your rank significantly. Learning vim is really worth it but if you're just getting started with cp you should first focus on solving problems and learning new algorithms because these are the primary things.