The libvpx package BuildRequires yasm for the optimizations on x86 and x86_64.
I tried to have a look, but it's clearly over my head. There seem to be enough incompatibilities between yasm and nasm (which I tried) to make a conversion complex enough so that I don't understand it.
Bug 603113 Comment 4 is a libvpx port to gas. yasm is no longer needed.