When visiting the Vatican Museum - is it possible to leave the museum for a lunch break and re-enter the museum after lunch? Or, is it a case of once you leave, you have to pay another admission fee to get back in? Thanks!
You cannot leave the museums and then re-enter. You would need to pay another entry fee if you left. There are two cafes and a cafeteria inside the museums so you could have lunch there if you wish.
Donna
What Donna said. Once you leave, you leave.
That's why the big cafeterias and snack bars and pizzerias are located inside...so you DON'T have to leave for lunch.
As others stated, you will need to pay to reenter the museums.
You put St. Peter's (Basilica, I am assuming) in the title of your post, so will touch on that as well. With the basilica, you can reenter (since it is free), but would need to go through security again. However, you cannot freely move between the two (with the exception of using the back door in the Sistine Chapel to get to the basilica, but this would be considered leaving the museums, so cannot go back without repaying and having to walk all the way around the outside of Vatican City to get there).
Also, if you do leave and decide to come back you would need to wait in any line that may still be outside. You won't get to bypass the lines again as you will with pre-purchased tickets.
As for the "back door" in the Sistine chapel, that is only for tour groups. They have guards at the door who usually check the tickets of everyone in the group to make sure there are no "extras".
Donna
As stated, this door is intended for the group tours, and they most likely have guards to make sure that only folks on the tours use this door. However, the one time I tried it (I was not on a tour), there was no one at the door to stop us and we walked through with no problems. I would not plan the entirety of your trip around this option as it does not always work out, but it is good to know about.
Having just visited the Vatican Museum a few weeks ago, I can tell you of our experience: We entered the museo and followed the "short itinerary" signs to the Sistine Chapel. We exited without understanding that voila! we were back at the St. Peter's Basilica and could not get back into the museo. So we walked all the way back around to the entrance (about one kilometer I think), explained our stupidity to the guards and they let us back in.
Insofar as lunch is concerned: There is a decent place to have a sit-down lunch inside the museum.