VISTA writes the FITS header associated with buffer 'buf' into a temporary disk file and then invokes the default editor to edit the file. When you exit the editor the modified version of the FITS header replaces the old version.
The default editor is vi, although you can specify any other editor by setting the environment variable VISUAL before running VISTA to be the name of the editor you wish to use (e.g. to use emacs, setenv VISUAL /path/to/emacs, where ``/path/to/'' is the full path to the emacs executable on your system).