I think V-Ray for Max 2015 should be ready soon, but that depends exclusively of ChaosGroup. The problem is the same for any RC version, we need an updated V-Ray SDK.
Besides that, there are or couple of bugs in Max 2015 that affect our plugins. We are waiting a fix from Autodesk. Sincerely we suggest not using it yet, as any new version it needs some maturity time.