The felt was used because of the masonite. However, with vinyl, you technically don't need it. You can use a house wrap.
But here's where the question lies. Will the mfgr. warrant the siding if you leave the felt on? If they do, and it's in good shape, I'd leave it. And as long as you have a smooth surface, that's good too. Check with the mfgr. to see what they say. (I say leave it if they say 'felt or wrap is ok'.)