The current win32codecs version in portage doesn't support VP6, although both mplayer and xine are able to use the dll (vp6vfw.dll). essential-20040704.tar.bz2 supports it, maybe someone could bump the version?
I'll try and get this into pre5 that is supposed to come out soon. I'll post more details here as they come in :).
I see mplayer-1.0_pre5 in portage, but no new win32codecs... :)
win32codecs now contains VP6 (see http://bugs.gentoo.org/show_bug.cgi?id=51492) I think this is solved :-)
Closing since VP6 is now in testing.