We were at St. Peter's/Vatican Museum 7/30. I think the morning St. Peter's hours in the guide book are accurate (can't comment on ending hours). We went in St. Peter's and climbed the dome, and then walked around to the Vatican Museum entrance in plenty of time for our 10:30 tour. Probably arrived at St. Peter's about 8:15 AM and there was no line for security or dome climb. I'm not sure what you mean by "other door". To enter museum or leave the sistine chapel? I think anyone who knows about it can leave the chapel through the "tours only" door, which gets you out of the museum and within the secure area outside St. Peters. We took an official Vatican Museum tour because it seemed like most other organized tours still involved waiting in the long line (it sure looked long the morning we were there). It seemed to me there was only one entrance, but with the official Vatican tour you got in without waiting in the line. Let me know if I didn't get to the heart of your question.