I've been doing all the maintenance and keeping record for our two CPO cars, I've only had to use the Dieselgate warranty not the CPO, and only once but I didn't have an issue. Granted it was unrelated to engine (battery replacement). But I asked both dealers we bought from at the time of purchase and they told me as long as I have receipts proving that I bought it and kept a record of date and mileage I would not have an issue. Just my experience!
That is what I figured but I have not been able to find any internet documentation on this. Looking at a Jetta and doing a bit of research before pulling the trigger!