Fraud is up with VW. In a sentence, the company designed pollution emission-control software to defeat testing by only working properly during testing, and otherwise not work to the same extent:
The VW cars under investigation emit up to 40x the national standard for nitrogen oxide, which is linked to asthma & lung illnesses.— U.S. EPA (@EPA) September 18, 2015
The Notice of Violation is here. As the NOV states, defeat devices are expressly outlawed. The letter also says VW was required to list all the emission devices on the car, so failure to list this software appears to be a deliberate omission. Another fun fact from page 4 of the NOV - even after being caught by outside experts with excess emissions, VW refused to admit what it had done until it learned that Californian and the EPA were about to disallow sale of 2016 models. The reason for disallowing is that the agencies had no way of knowing the 2016 models wouldn't also fail, until VW then admitted what it had done with prior year models. Media reports that VW admitted its action after being caught are actually making the company look better than it was. I'm also curious about the delayed public response by the company, and wonder if there's more out there where they haven't yet been caught.
American environmental law allows for criminal penalties, usually limited to when someone intentionally committed the illegal act, and sometimes further limited to when someone intentionally committed the illegal act with the knowledge that it was illegal. Either way, multiple people should be in trouble at VW. A list of criminal provisions under the Clean Air Act are here: false statements, tampering with a monitoring method, and knowing failure to report at a minimum seem applicable. The NOV, sadly, doesn't say anything about potential criminal penalties. Hopefully that's still to come, and not just symbolic criminal prosecution of the company but of the actual people involved.
Regardless, as the first link shows, there will be lawsuits by the public.
So, differences from Exxon's early knowledge of climate change? The scientific elite knew what Exxon knew in the 1970s and 80s, so that's different, but that doesn't matter a whole lot. Fraud is still fraud even if the defendant argues the victim should've been able to figure out the truth.
The more pertinent difference is that Exxon funded other organizations that said things Exxon knew were untrue, while VW has just been doing the untrue acts directly. Tying Exxon's responsibility to the denialist statements by organizations it funded would take a little more work. More on this later.