When the original 54.5 mpg CAFE proposal was announced (which will actually be around 40 mpg in the real world), Volkswagen did not sign on to the agreement. The reason is that VW says the current plan helps U.S. automakers by being lenient on big pickups but doesn't have much love for modern diesel vehicles.
Make that "enough love," since the EPA says the rule does give "credits for technologies with potential to achieve real-world CO2 reductions and fuel economy improvements that are not captured by the standards test procedures." According to Reuters, since August, Volkswagen America's general counsel and VW representatives have met with the White House and "transportation and environmental regulators" to express the company's concerns. We haven't heard that VW's efforts are bearing any fruit, but that doesn't mean we won't hear more about them in the future.