pdath: You have missed the important bit that VW has already admitted it. So this is not a case about proving they did - but about the damages to be awarded and how to put it right.
What have VW actually admitted to. Putting software in the ECU that ensures the emission standards are met under certain conditions?
Fredd99 has quoted part of the EU regulations and read in isolation it would seen VW could not be deemed to be compliant. However until you read the complete regulation and interpret how that passage is applied nothing can be taken for granted. There's a difference from complying with the rules as written and complying with their intent. It's possible to comply with the rules as written yet still not comply with their intent. This is where smart lawyers come in.
I'm playing devils advocate to those who are predicting the end for VW.
Let's say you've built a new engine and tested it for compliance against the rules. The rules say that the emissions shall not exceed certain levels as tested at certain RPMs. These RPMs have been chosen because they are thought to be representative of normal operating conditions and the expectation is that if the emissions are within tolerances at these RPMs the engine should be able to meet compliance at all other RPMs.
Your testing shows compliance.
At some later time you are doing some other testing and discover that for some reason your fabulous new engine isn't meeting the emission standards at other RPMs.
Are you compliant with the rule as written? I'd argue that you were.
Are you compliant with the intent of the rule? I'd argue that you weren't.
What VW has done was deliberate not by accident which is certainly a lot worse that it happening inadvertently.
Depending on how the rule is interpreted VW could possibly argue they still complied with the rule.
There's plenty of cases where smart lawyers have won cases on these sorts of technicalities where the defendant was as guilty as sin.
Also I'd bet VW aren't alone they've just been "unlucky" to have been caught. There'll be other manufacturers out there with hearts in their mouths hoping they're not caught out on this or some other rule. There'll be a bit of a song and dance but I wouldn't be surprised VW will pay some token fine and the event will be forgotten as there will others who won't want to much blow torch action on this subject.