For a slower pace, split them across two days. Check current opening information instead of relying on an old itinerary.
Vatican City · YOUR QUESTION, ANSWERED
Can I visit St. Peter's Basilica and the Vatican Museums in one day?
Fix the museum reservation first, then build the basilica around it. Enter St. Peter’s at 7:00 a.m. for Mass or prayer before the lines form, walk the 15 minutes around the walls to the Vatican Museums entrance on Viale Vaticano, and allow 2–3 hours for the route to the Sistine Chapel. Alternatively, take a morning museum slot and return to the basilica in the afternoon; it stays open until about 7:00 p.m. in summer and 6:30 p.m. in winter. Leave a generous gap for security, food, and changes in access, and avoid Sundays, when the museums are closed except on the last Sunday of the month.
Sources & further readingSt. Peter's Basilica: visitor questionsVatican Museums: visitor information
Official sources cover institutional information. Suggested pacing and itinerary choices are planning guidance. Check current arrangements for your travel dates.