>From Vim's point of view, the entire GPL'ed code constitute an
addition (a special case of a "change"), so it is all subject to the
conditions you apply to changes.
If you want to exempt, say, the addition of library code from your
conditions on modifications in general, you need to add language
in the license text that says so explicitly. (And doing so may not
be a trivial task if you do not want to introduce loopholes that
allow people to disguise essential part of their contributions as
undisclosed library code).
A case like this has to be seen from both sides: to combine the two
modules, you have to satisfy the license of each one.
>From Vim's side, the question is: does the Vim license permit linking
it with that library? From the library's side, the question is: does
the library's license permit linking it with Vim? The questions are
symmetrical, but since the licenses are different, the answers are not
symmetrical.
If the current Vim license says no to the first question, adding text
to the Vim license could make it say yes in the same circumstances.
But there is also the second question. If the library's license is
the GNU GPL, it says yes if and only if Vim has a GPL-compatible
license.
