Well, ex is 'ex'tended ed, and vi and ex are the same program.
From 1976-1979, the way to open vi was to execute 'ex', and run the 'vi[sual]' command in ex. In 1979, ex learned to launch the 'vi' command automatically if argv[0] is "vi", and a symlink from 'bin/vi' to 'bin/ex' was added.
From 1976-1979, the way to open vi was to execute 'ex', and run the 'vi[sual]' command in ex. In 1979, ex learned to launch the 'vi' command automatically if argv[0] is "vi", and a symlink from 'bin/vi' to 'bin/ex' was added.