As far as I know preps/sups/spells that give resistance to fire/cold/mental(and more) increase your chance to save against the spell, meaning half damage...or not so effective song.
Protection against different things give damage reduction against that certain damage type.
Good examples are the rangers resist heat/cold vs. a druid protection against elements.
Regarding vulns, I belive there wont be double vuln. A wrathing silver sword wont do double-double damage.
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