The xvmc USE-flag is used xine and mplayer and enables support for the X-Video Motion Compensation extension. This extension is provided by the x11-libs/libXvMC package, and neither installed by default nor depending on the USE-flag.
Created attachment 80914 [details, diff] Patch to make xorg-server aware of the xvmc USE-flag
No, mplayer/xine-lib depends on x11-libs/libXvMC with modular X installed. If you don't get that dependency installed, then you are doing wrong stuff wrt virtual/x11 definition in /etc/portage files (package.provided, virtuals) and you have much bigger problem than just this one lib.