Actually, I think the reason they added the level requirement to the bard songs is so that people could NOT take levels in Virtuoso and still advance their bard songs. After all, bard songs are BARD songs; Virtuoso develops skill in their own song list. If I could use Virtuoso levels toward Bard songs, that would be having my cake and eating it too. All I'd pass up would be a REF save and a +1 in Bardic Knowledge. In fact, it was the prospect of getting so much and losing so little that was the reason I was planning to do it. Now there would be a real cost to multiclassing.