Konrad,
I haven't heard anything regarding the Viewliner diner, but Amtrak did plan to give it a full overhaul including new trucks I believe. Additionally, part of the reason for bringing it back was to make it serve as a test bed for the new dining cars that hopefully will be on order soon.
The Viewliner cars are modular cars, in that they remove a big panel on the side of the car and just slide the rooms and/or furnishings into the car. They're then bolted down, have power and plumbing as needed connected to the module. So the single diner was supposed to serve as a test bed if you will to ensure that all the modules being built would actually fit properly into the car. It is possible that they hit a snag and something didn't fit and they had to go back to the drawing board. Or it could be that they just didn't get the modules built quickly enough.