One of Osiris’ Reflections says this in Beyond Infinity:
We must find and stop Panoptes, but none of us have discovered a timeline where we succeed. This is what it looks like when we fail. This is the future the Vex want.
Neither Light nor Darkness exists anymore. The sun’s warmth is gone. All life… lost.
Given Osiris’ insistence that Panoptes be destroyed, it seems like- at the very least, in his opinion- that the Vex are directly responsible for the Simulant Future. The Darkness couldn’t be responsible for this future, because the Darkness no longer exists. That’s the Vex’s collective goal, as Osiris says in one of the Kairos Function entries:
But Darkness is not merely absence of Light. Darkness is an entity unto itself. Put simply, Darkness is not Nothing.
But the Vex? The Vex seek neither Light nor Darkness. They seek Convergence, the reduction of all life to its simplest, most meaningless form. An entelechy of zeros and ones.
Given that the Darkness isn’t responsible, and it’s almost certainly not the responsibility of the Light, that leaves the Vex.
That’s from Null Calamity 9:
Osiris hovered, cross-legged, at the center of a ruined Lighthouse. A dead sun loomed in the lightless sky. Arrayed on the ground below to the horizon, yellow Vex eyes pierced the gloom.
Sagira zoomed straight to Osiris’s side. "I thought we averted this future!”
Eyes closed, Osiris shook his head. “Many equations lead to this answer.”
The ‘equation’ that we stopped involved Panoptes, but not every possibility that led to the Simulant Future required Panoptes, and as a result the Vex are still able to cause the Simulant Future if they aren’t consistently prevented from doing so. We cut off one head, but there are still more.
It’s left fairly vague, but Osiris does say this:
Compartmentalization. Isolate the pieces of a network, so that 1) each subnet may operate independently, and 2) any harm that befalls one subnet will not necessarily befall the rest. The Vex learned this lesson well. Many subnets, many equations, all executing toward the same answer: convergence. They gambled that eventually, one of their subnets will achieve it. Speaking in purely mathematical terms, it’s a very safe bet.
The Vex seemingly brute-forced their way through the problem, as they had the computational power to run infinitely many tests with an infinite number of variables. Using the Forest and the simulations within it, they were very likely to come across a feasible Simulant Future eventually.