Another problem that I'm worried about is that many people will think Vim _is_ GPL. It will be mentioned in lists in magazines and on web sites. We would have to check and request correction where it's wrong. Perhaps it would help to give a good name to the dual license. GPL++ perhaps?
You could give the other license a name, like VimPL, and describe the disjunctive dual license as VimPL | GPL. GPL++ as a name would be misleading, since it implies a successor to the GPL.