Unfortunately you cant't implement your own VSTs in Sibelius 4. I'm having the same problem and I think upgreading to sibelius 5 would just be the simplest way. I didn't konw what a midi yoke is, too.
So I googled it, but It seems quite complicated and unusual to me (As far as I understood it you redirect the midi data coming out from your sequencer to your vst instrument.. is that right???) and you should consider that you would probably have a much easyer life directly implementing the instrument to sibelius 5 where thats possible.