So, those two different contingency plans for the Fall of Terra
Guilliman is a symbol of the system. He is bureaucracy itself, law and decree. Grounded in reason and pragmatism, he plans and manages everything systemically.
And one thing about a system is that it is fundamentally based on the continuity. System doesn't die alone; rules, regulations, stability, protection, you name it―things built upon a system collapse along with the system when it falls.
That is why Guilliman declared the Imperium Secundus when he lost contact with Terra. He could not risk the system outage, as such discontinuation could and eventually would lead the whole Imperium (or whatever left of it) to founder.
So he had to sustain the system. Preserve it, for a system is unlike a candle; it cannot be easily rekindled, as once the flame goes out the entire pire and the brazier collapse upon it, burying everything beneath the wreckage. Here it is crucial not to let the flame die out, and Guilliman knew it well.
‘Ultramar is all that we know we have. If the Imperium endures, then we will re-join it when the storm dies, but if it has not endured elsewhere, then we have preserved it here.’
So the system, the Imperium was an object of preservation, not succession to him. This point was explicitly stated this in Guilliman's conversation with Lion:
‘You are establishing a second Imperium on the corpse of the first.’ ‘No. I am tring to keep the flame alive.’
Frankly, the foundation of the Imperium Secundus had been quite hard to fully swallow for me. I got the idea but it still didn't sound too logical or reasonable to me, and that always bugged me.
Like, okay but did he really have to do that? Was it really necessary? Necessary enough to risk TREASON, when they are already facing one and fighting against it, in this era of Heresy?
Now I think I get it; Guilliman's reasoning to the Imperium Secundus was like one of a server engineer, I guess.
Think of him as a local branch manager. When his server lost connection to the main server/hub and he couldn't hear anything from the HQ, Guilliman did that one thing any engineers in his shoes would have done; he made a backup copy out of his branch server to prevent the total loss of the OS that runs the entire system AND the company. Guilliman knew that all the legacy code would be lost along with the OS if he hadn't, and without them rebuilding a functioning OS would be practically impossible.
So yeah, now I could see that he did what needed to be done.
And there is Vulkan, the myth incarnate. His story, typical monomyth, follows deeply mythic narrative in multiple levels. Most primarchs do share some of heroic narratives yes of course, but Vulkan stands out as a perpetual, whole ordeal around him being about death and rebirth.
The ancient Promethean creed told that the circle of birth and rebirth not only maintained the balance of nature, but also held the belief of life eternal, of resurrection. (…) The sacrifice of one could bring about the apotheosis of another.
The cycle of death and life continues in a loop, yet it differs from the 'continuity' a system requires. Resurrection here is not about persisting beyond death; it is the death and end of the old and the birth of the new. It is not the same after the death. The subject is not preserved, but the new succeeds the old.
This actually matters, that nothing that dies can ever come back the same; so important this theme is that it tis mentioned and emphasized multiple times throughout DF and OE.
'Nothing that comes back can ever truly be the same, so says the Circle of Fire.'
He remembered part of an old Nocturnean proverb about the dead. That they could never come back. Not the same. It boded ill.
'Is it him?' asked Abidemi as they drew nearer and all the finery of the primarch's craft and artisanal skill became apparent in his armour. Zythos frowned as he turned to the other. 'Who else would it be?' 'No,' uttered Abidemi, his tone suggesting to Zytos a question meant for long before this moment, 'is it him?'
And? I think this is the most significant and fundamental difference between Vulkan and Guilliman.
Vulkan embodies the resurrection by fire and sacrifice, thus knows that a new flame can ignite even upon the ashes of everything that has burned away. This was the reason why the Emperor chose Vulkan of all primarchs as His 'backup plan', to task with a heavy burden of burning down the Throneworld to ashes should He fails.
You will never know how much i dig it. I find it greatly amusing that big E's one and only option, the best one he had, is destined to rely on a very mythical mechanism of death-sacrifice-rebirth when His Imperial Truth denies and even forbids myths, superstitions and all such things.