Change the default text editor in Terminal

Simply set the EDITOR variable to vim in your bash startup file.

EDITOR=vim

From the bash manual

   edit-and-execute-command (C-xC-e)
      Invoke an editor on the current command line, and execute the result as shell commands.  Bash
      attempts to invoke $FCEDIT, $EDITOR, and emacs as the editor, in that order. 

This question gets to the root of how you want to work - so there's some "preference" that may be in play. If you come from the school of vi you might prefer a command as opposed to the emacs key bindings.

I prefer using fc since my big chunk of time post-emacs was spent in Korn shell (ksh) and I used fc built in to summon the editor. On all macOS systems, by default vim is chosen over the emacs command.

For changing the editor, I usually set FCEDIT so that it catches both the bash / emacs invocation for the fc built in commands. I rarely change things, though since it's nice to know I can get directly to emacs when needed (on occasion it's better for me) and let fc do the heavy lifting for me. So if I were to head back to “emacs land”, fc is how I'd get there initially.