Possibly you have $VISUAL set to vim as well, and vipw/vigr (at least the ones in the passwd package on Debian) uses that instead of $EDITOR if both are set (like most applications do), so:
VISUAL='vim -u NONE' vigr
Should do it as long as your vipw/vigr like Debian's expects that to be sh code (runs sh -c -- <that-code-concatenated-with-" /etc/group.edit">) instead of the name or path of an editor executable (to call with /etc/group.edit as argument)¹.
From the Debian man page for vipw/vigr:
When looking for an editor, the programs will first try the environment variable $VISUAL, then the environment variable $EDITOR, and finally the default editor, vi(1).
¹ Most of the time, $VISUAL/$EDITOR are intended as code passed to sh (that's what Debian's environ(7) man page specifies). In my experience, other possibilities are code passed to $SHELL (see also less which upon v runs sh (possibly via system(3)) to run $SHELL to interpret the code), or the program will do a shell-like tokenising of it, skipping the call to a shell and avoid doing other forms of shell expansions/interpretation, or expect the name or path of an editor only and don't run any shell (for mailx, POSIX specifies $EDITOR is meant to be an editor name, and $VISUAL an editor pathname, I doubt many implementations honour that).