As someone with programming experience I'm not sure what would take so long to implement such a system. Seemed like a pretty basic model override system, after that its just designing the interface. Even the combined piece armor would be fairly easy to work around, either requiring you to fill all vanity slots that are needed to fully override it or making the unfilled slots display as empty.

Outside the commercial developer red tape this is something a single coder would only need a day or two to put in place from scratch. Its perfectly reasonable to request it be pushed to 2.1 or at least an early hotfix after. There is no practical reason why Vanity needs to be delayed until 2.2, especially giving the demand for it.