Dust is not Vulnerability. "essentially" is not exactly the same. They are different things, and of course they stack, they do different things! One gives vulnerability stacks, the other lowers PRR (which results in one doing more physical damage, but not more magical damage). Vulnerability is a multiplier to your damage. Saying they're the same thing is along the same logic as saying increasing your strength is the same thing as they both result in doing more physical damage.