In theory at least, the Ogg file-parsing code should be shareable with the Vorbis playback stuff, but I don't think the STB codec exposes it at that level.
You could conceivably use liboggz + libfishsound (both from Xiph) to make a generic anything-in-an-ogg-container decoder that would support Vorbis, Speex, and FLAC all in one. See
here for an example.