I've set nobackup and nowritebackup per this question - VIM destroys symbolic links under Windows
However, whenever I modify the file via the link, the target file is set to read-only.
How can I prevent the link target from becoming read-only?
This seems like a bug in VIM incorrectly interpreting file-attributes on Windows.
The bug is described in 'backupcopy' and Windows Vista symbolic links, where a patch was developed to solve the problem.
I believe you will just have to wait until that patch finds its way to the release. Your other options are to download the latest VIM patch (if later than your version) or to compile your own, and if the bug is still not fixed report it again to the VIM developers.