I have a planned vacation during the second week of orientation

I received a job offer today (front desk receptionist, nothing prestigious but a great offer for the position) which I enthusiastically accepted. The problem is that I have a family vacation planned with my elderly parents that overlaps with the first two days of the second week of training.

Should I cancel my vacation in order to make the best possible impression, or is there an artful way to let them know I had made plans that would interrupt the two days of training? I am treading lightly, and want to maintain the positive impression that I've made.