I'm a bit confused about the role played by the city of Rome in the Year of the Four Emperors scenario. It doesn't seem to be all that important to hold it when the victory points are calculated at the end.
A couple of days ago I finished the scenario as the Otho faction and managed to keep Rome from beginning to end, along with all the other Italian victory locations. I doubt I could have done much better with the limited forces Otho has at his disposal, but Vespasianus still won the game in points, even though his forces never came closer to Rome than the borders of northern Italy. The Vitellius faction was almost wiped out by my armies, the barbarians and Vespasianus, so he didn't matter in the final score.
So I just pretended I had actually won the civil war and there was no Flavian dynasty of emperors.