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what if the forces of good failed to stop vandire and his goons on terra how different would the imperium be now?

Due to Vandire's incompetence, It probably would collapse over the course of a few hundred years into countless small empires which would be easily picked apart by Chaos, Orks and Eldar.

I would imagine civilisations would cluster around powerful naval bases, the more sensible space marine chapters and remaining Primarchs.

It's likely that the traitor legions would be able to establish an empire (or several very loosely allied empires) around the Eye of Terror.

His actions were erratic and illogical, even for something as bloated/rotten as the IoM, I think things would basically just collapse or hundreds of sectors would secede/rebel and form their own versions of the Imperium, personally I think it's all redundant anyway as somebody else if not the proto-SoB would go "frak it!" and assassinate him eventually. The state of the Imperium, and consequently how it would be dealt with afterwards would depend mainly on just how far Vandire had been allowed to go on with his lunacy up until then.

It would have been ripped apart in civil war, likely leaving several fragmented 'mini-Imperiums' all feuding with each other.
Whether the enemies of the Imperium would be able to gain a foothold depends on how much the infighting would weaken the Imperial factions. If the resulting wars would be devastating, it is quite likely Chaos warbands might have been able to establish empires of their own. Xeno empires might also be able to spring up more often, adding to the overall chaos and warfare.
Basically, it would be the same except with a lot more different factions that'd be more equal in power, and therefore there'd be even more war and grimdark.

At the end anyway he was using machinery to stay alive and pretty old. By time good guys where turning on him he was older. He would be older when he rose to rank of high lord.

So he,s gonna be in later stages into his life anyway. How long could he have lived even if they did not intervern.

Given Mars was turning against. If they refused to back him they could hrind everything to a utter halt easily.