>if I have an iron sword that's also poisonous bite does it
>still do vuln damage to Elves...

Yes. (What would be the alternative? It only does extra damage to elves if it has an 'iron pwning' verb?)

>Or what if it's iron and something like defilement. If it
>still does vuln damage and they have up resist negative, does
>it do regular damage or do they resist it alltogether?

It's a wash, roughly.