Please take note of the following:

From an official Vatican website page on Guided Tours:
(link is here under Useful Information)

"Visitors not attired properly will be refused admittance."

And from vatican-tours.com :

"Please take note that there is a strict dress code for St. Peter's Basilica and the Vatican Museums. Women must cover their shoulders and knees and men must wear full length pants.  There is actually a fashion check-point and if you are not dressed accordingly the Vatican staff will refuse entry. Please keep this in mind when planning your tour  to the Vatican."

(highlight ours)