When you type in vi [filename], the [filename] will bring up the file, "filename". If there isn't currently a file by that anme, it's create a new file by that name. You can't edit an old file that doesn't exit, you in turn will create a new file if the file doesn't exist. You can check to see if a file exist by typing, ls [filename].
If you have a command called adduser as you mentioned, you can look to see where that command is located by typing, "type adduser". If it's in your path, you'll see where it's located and you will edit it either by typing in the full pathname of the file, or by changing to that directory to edit the file.
-- L. James
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