Yes, you can use them at face value at any restaurant and top up the extra in cash.
Incidently you can also mix in the HB restaurants, eg 3 of us ate from the set menu at Silver Spur, but one son wanted items off the A La Carte and it was no problem.
I think the restaurant CM's have a better understanding of the system than anyone else, we were given incorrect information at City Hall on one of our trips about the vouchers use.