I want to add LICENSE=foo to pkgsrc/editors/vim-share/Makefile.common
  but what should it be? Do we need to create a new license?

  The README.txt file says this:

  COPYING

  Vim is Charityware.  You can use and copy it as much as you like, but
  you are encouraged to make a donation to orphans in Uganda.  Please read
  the file "runtime/doc/uganda.txt" for details (do ":help uganda" inside
  Vim).

  Summary of the license: There are no restrictions on using or
  distributing an unmodified copy of Vim.  Parts of Vim may also be
  distributed, but the license text must always be included.  For modified
  versions a few restrictions apply. The license is GPL compatible, you
  may compile Vim with GPL libraries and distribute it.

That doesn't explicitly grant the right to make derivative works, and
I'd guess hasn't formally been approved by FSF/OSI, so I'd put that text
in licenses/vim-license and point at it.  This is particularly true
because it says 'unmodified copy', which makes it seem non-free.

Typically a package with its own license needs a new license file.