Mutual vulnerability is how the framework locates a foothold for cooperation in a scenario where trust is absent. Rivals who agree on nothing else may still agree that certain outcomes are catastrophic for all of them, and that overlap — not goodwill — is what has historically made arms-control agreements possible. The framework treats it as realism rather than idealism: you do not need friendship, you need overlapping fear, plus a clear, shared map of what no participant can survive.
It is the motivational counterpart to the shared-viability constraint: the constraint states the rule (no advantage by degrading shared conditions), and mutual vulnerability supplies the reason a rival might actually honor it. Its limits are obvious — overlapping fear produces narrow, fragile agreements, not comprehensive ones, and it works only where the shared danger is legible enough to be believed — which is why it pairs with verifiable pacing rather than standing alone.