Volkswagen Tiguan 2009: What is causing the vibration when accelerating?

My four-and-a-half year-old Volkswagen Tiguan has developed a vibrating issue when you accelerate. The dealer has had the car now for more than three weeks and has replaced the injectors and the diff without fixing the problem, but has called in VW to help. So far I am satisfied with what has been done, but what should I be paying for, given the length of time they have been working on the car and the parts they have replaced which haven't fixed the problem. Should I only be paying for the parts that have actually fixed the problem? Also what if they can't fix the problem or don't want to invest any more time in trying to resolve the issue, what options would be fair and reasonable in this instant.