Mike,
From what I've read and what we've tested, the Vizia RF and Vizia RF + devices are fully interoperable. There should be an upgrade available soon for the Vizia RF remotes to setup the new devices if you can't do it already--and of course ThinkEssentials 2.1 will let you set up and control the network. If you mix the original line and the new line, you lose the benefit of a totally-40kbps network and of course the original line didn't have some of the cool new features, but you should be fine.
Also, on 3-way circuits, I would use the same design on both sides if you're using the new Vizia + matching/coordinating remotes.
Chris
ControlThink