Re: draft for new Vim license
This is wholly satisfactory to me, at least. To address one of your
other concerns, I don't think it would hurt to add the following
"You are encouraged to license your changes under the Vim license as
well, and submit them to the Vim maintainer for possible inclusion in
future versions of Vim."
This is not legally binding, but it does nudge people in the direction
you want them to go.
This is a good idea. In situations like this the FSF urges people to
cooperate with the maintainer.