I initially ran into quite a lot of difficulty building libvpx (webm) for windows (win32) under msys or linux using mingw32, but I eventually cracked it.
git clone git://review.webmproject.org/libvpx.git cd libvpx CROSS=i686-pc-mingw32- ./configure --prefix=/usr/i686-pc-mingw32/mingw --target=x86-win32-gcc make make install
The only thing you would generally need to change is the “i686-pc-mingw32″ generally needs to be changed to reflect the mingw in your build system and the prefix needs to point to your mingw toolset root. Not only will this create a working library to use within windows projects (the libvpx.a can be added to visual studio projects), it creates and installs a library that builds perfectly into ffmpeg win32 builds.
Simple, no patches required.