Once the formal entrance to the emperor’s apartments, this large round room has an open roof once covered by a dome. No decoration other than around the doorway but lovely brickwork and well preserved.
Located just a few seconds walk away from the main area with the Emperors Peristil and also the cathedral (go up the steps to Vestibule rather than down into the vaults). All the things to see here are all within a few seconds walk of each other. An entrance to the substructures of Diocletians Palace is here too and the ticket office where you can buy tickets to the church etc. The Triclinium is maybe a 1 minute walk, more depending on how bad the crowds are!
It is recommended to spend a couple of minutes in the vestibule, you won't need any more than that to have a look around and take a few photos. Apparently the acoustics are incredible here and you might just be lucky enough to hear ‘a cappella’ singers performing, but we did not. We visited during the day and also at night, when it was a little quieter, but always very crowded.
You only really need half a day to explore and see all the sights of Diocletians Palace and Old Town, but we would recommend to return again on an evening to see the streets lit up and for a completely different experience (although sadly often just as crowded!)