When I went to the Vatican last year I took a bus tour via Amex. I did notice more than one group of private tours (4 people or so) cut in the line between bus groups and no one seemed to notice or complain. We took about an hour in line and they probably waited 10 minutes. For me not counting the cutting issue, I realized later a 4 or 5 person private tour group would have been the same or less price than 4 or 5 tour bus tickets. YMMV