Why it's tight, not impossible

The Vatican and the Colosseum aren't within walking distance of each other, and both deserve at least 2–3 hours to see properly — cramming both into one day works, but it's a genuinely full day with little slack for delays.

A workable schedule

  • 8:00–11:00: Vatican Museums and Sistine Chapel
  • 11:00–12:00: St. Peter's Basilica (a shorter visit than a dedicated Vatican day)
  • 12:00–13:00: Travel + lunch near the Colosseum
  • 13:00–16:00: Colosseum, Roman Forum and Palatine Hill

If this sounds exhausting rather than exciting, consider spreading it over two days instead.