Typing :r in vim gives the file name. Replacing r with other letters gives extensions. This seems reminiscent of the top answer mentioning filename modifiers. The vim extension mainly relates to script files used by the Vim text editor to configure and extend it. These default vim scripts are found as scripts from Vim. Files with .vim extension contain configurations and customizations for the Vim text editor. Vim recognizes file formats that determine line endings removed when reading or added when writing files. This can cause issues displaying characters or running scripts.
To open .vim files, use Vim text editor. They likely cannot convert to other formats. Each file edited triggers Vim to recognize the file type and set ‘filetype’ accordingly. This triggers events to set syntax highlighting, options, etc. Filetypes and ‘compatible’ do not work well together since Vi-compatibility means global options.
Vim Editor
Neovim
MacVim