Quote Originally Posted by Yfrwlf View Post
If vbox uses Wine libraries then they have to [contribute patches back to Wine] by copyright law.
No, they don't. Per the GPL, they need only provide corresponding source code alongside any binaries that they distribute (or provide the "written offer to distribute", which seems to not be used very often); there is no requirement that they send anything back upstream. Stallman has even criticized such a requirement in other licenses.