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I know that a critical hit, which deals double damage, immediately causes an unconscious creature to fail two death saving throws. Vulnerability also causes a creature to take double damage against certain attacks.
Does taking damage from a type a creature is vulnerable to cause two failed death saving throws as well, or does it simply have a higher chance of killing them instantly (if the target is damaged equal to or greater than its hit point maximum)?
The rule for taking damage at 0 HP is:
Damage at 0 Hit Points. If you take any damage while you have 0 hit points, you suffer a death saving throw failure. If the damage is from a critical hit, you suffer two failures instead. If the damage equals or exceeds your hit point maximum, you suffer instant death.
The two failed throws comes only from a critical hit not from other forms of increased damage.
Critical hits and vulnerability are two separate mechanics with two separate names and that function differently, and there is no reason to conflate the two.1 If the death saving throw rules were meant to include them, they would have listed vulnerability by name in the description.
Of course, because the damage to a vulnerable creature is increased they do have a greater chance of suffering instant death, but they would only take one failed death saving throw assuming they survive.
1 - Vulnerability actually doubles damage, but critical hits only has you roll the damage dice twice. It is a subtle but important distinction, but critical hits do not double the damage.
