The reason and point of the vision would be to show that God had conquered death through his beloved and all demonic powers fell down and worshiped him, and the Beloved. The salvation of the visionary faithful is assured. No more struggle. That's the vision -- a message of total victory.
Maybe this is getting somewhere.
There are a lot of claims about Belial in the War Scroll, specifically about defeating Belial. In the War Scroll we see a lot of proactive prophecy, i.e. many of the prophecies/visions in the War Scroll are not retroactive narrations of events that have happened, they actually imagine what the future will be.
War Scroll:
Col 13:
You appointed the Prince of Light from of old to assist us, for in His lot are all sons of righteousness and all sprits of truth are in his dominion. You yourself made Belial for the pit, an angel of malevolence, his dominion is in darkness and his counsel is to condemn and convict. All the spirits of his lot, the angels of destruction, walk in accord with the rule of darkness, for it is their only desire. But we, in the lot of Your truth, rejoice in Your mighty hand. We rejoice in Your salvation, and revel in Your help and Your peace.
It goes on, talking about how the Sons of Light will defeat the Sons of Darkness.
Blessed is Your name, O God of loving-kindness, the One who kept the covenant for our forefathers. Throughout all our generations You have made Your mercies wonderous for the remnant of the people during the dominion of Belial.
etc., etc. So, the War Scroll, and other writing from Qumran, present visions of how God will defeat Belial/Belair/Satan. They identify Belial as the "Lord of this world".
Vision of Isaiah seems much closer to the War Scroll to me than any canonical material. It uses a lot of the same language and has a more similar worldview.
The War Scroll foresees how God will aid the Sons of Light in fighting against the "forces of Belial" and how God will defeat Belial himself.
...and in the seventh lot, when the great hand of God shall be lifted up against Belial and against all the forces of his dominion for an eternal slaughter...
Is the Vision of Isaiah a competing view to these visions of war? The War Scroll talks about how this will need to be accomplished via a massive conflict. But the Vision of Isaiah says that Belial will be defeated via trickery, without need for a conflict.
Could the narrative of Vision of Isaiah have preceded the First Jewish-Roman War, being a competing vision in opposition to those stoking violence?
I guess the main question is, does the Vision of Isaiah require a recognizable event or can it have been written in a manner similar to the War Scroll, in which it is projecting something that had not occurred? Many prophecies and visions were retrospective narrations of things that had occurred, but not all of them were. So would it make sense to write a vision that talks about the descent of the Beloved of God (the Prince of Light) who would trick Belial into executing him so that he could descend into his realm to defeat him, if no such event had yet occurred?
I think many assume that such a vision can only be a retrospective account of an actual execution or as a response to pre-existing narratives about Jesus.
Vision of Isaiah:
And we ascended, he and I, upon the firmament, and there I saw the great battle of Satan and his might opposing the loyal followers (honorantiae) of God, and one surpassed the other in envy. For just as it is on earth, so also is it in the firmament, because replicas of what are in the firmament are on earth.
And I said to the angel, `What is this war and envy and struggle?' And in reply he said to me, 'This is the devil's war and he will not rest until He whom you wish to see comes to slay him with the spirit of His virtue.'
But could the earliest version of the VoI narrative not have simply developed on its own as an alternative vision for the defeat of Belial that did not rely on war?
Alternatively, could the Vision of Isaiah have been written following the war, providing an alternative hope for the defeat of Belial, after the war had failed.