My understanding is that programs must have information regarding their vacation offerings in their contracts. They must also provide clear policies regarding vacations. Furthermore, the vacations must comply with local/state/federal laws. I don't believe their are any other requirements.
I believe that if they do not state that you have to take tests during vacation that you could complain, but good luck with that. You won't endear yourself with anyone for it and I have a hard time believing that the ACGME would make a big deal out of it unless it was a systematic and very egregious problem. It isn't ideal, but medical training, despite the work hour reduction, still requires a lot of personal life sacrifices.