The name "Vis Uban" translates as "Place of Great Power." A reference in some partial translations Daniel had made from the Library of the Four Races spoke of a city of the Ancients called Vis Uban which was going to be the crown jewel of the entire Ancient domain, however the plague which wiped out the Ancient race began there while the city was still under construction. In searching for the Lost City, Jonas relied on an Asgard translation of early Ancient which suggested that the "Lost City" may in fact be the "City of the Lost," and that Vis Uban, as a city that was never completed when the Ancient civilization fell, could be the city they sought. He further theorized that its address could have been the last of the list of gate addresses that O'Neill had entered into the dialing computer when he carried the knowledge of the Ancients.
Following Jonas's theory, SG-1 traveled to the planet, which may have been designated P4T-3G6, and they found a world of ancient ruins with writings in the oldest known dialects of the Ancients and inhabited by a simple nomadic tribe. It was on this planet that Daniel had appeared after being returned to human form by Oma Desala, and where he and his teammates were reunited following his ascension. Shamda, the leader of the nomadic tribe, welcomed SG-1, and cooperated with them in relocating his people to another planet so that SG-1 could use the ancient ruins and a fake tablet to lure Anubis as part of their mission to destroy his superweapon. It was never determined for certain if the planet was indeed the Vis Uban which the Ancient text described, but Daniel later ascertained that the earlier translation was in error and that the Lost City was indeed a "Lost City", a place that had been made lost, or hidden to those who would find it.
Episode Reference: Fallen