Under union workrules, can Amtrak even have the diners assigned to other trains and open for use without a food service worker present?
I get that people don't want to see new cars sitting in a yard, but that cost of ownership is money already spent. You're not saving any money by using them. If anything, you're probably wasting it.
In the yard, they're not breaking down requiring service, they don't need to be cleaned, and they aren't causing incremental fuel burn.
Putting them into service on a state service elsewhere simply means they're not available to swap into service when they're needed for another car that's out of service, and they're now a new type of car that the mechanical guys will have to deal with and won't necessarily have the parts to repair with if something does break... Sure, there's commonality with the other VL2's, but not necessarily with the interior fittings, and those are the things that will break if they're opened to the public.