[RPG] How does Vulnerable work with the same Resistance

2010 GenCon Preview mentions The Ravenloft Roleplaying Game in Q4 2011, (it's mentioned at 58:00, and some questions are asked a couple minutes later) it's a separate RPG, but Fully Integratable with D&D4e, so I suppose that does count as campaign setting.

The character takes 0 damage.

Being subjected to 3 instances of 5 cold damage does not stack, and each is resisted.

Note - I'm assuming you are referring to the Chillborn Zombie ^DDI as it appears to match your description.

The Chillborn Zombie was created when the rule was that a creature was only subjected to one instance of damage from overlapping auras. That rule was changed in May 2010. When created, the zombie needed an exception noted in it's aura. It isn't needed since the rule change, and there is no need to grant it a further exception.

This change removes the rule that prevents stacking in damage auras, ensuring that monsters are achieving correct damage output

There is a discussion of this change on the wizards boards, with clarification from customer service.