Some people said that the Dunmer fire resist does not stack / help for vampire and a ring with Fire resist is much better.

I just have some thought, maybe the Vampire does not lower your fire resist but simply x1.5 for fire damage, thats why with multiple fire resist gear and being Dunmer wouldn't help at all since you might already reach the soft cap.

EDIT: Basically what I'm saying is, most people thought it is an addition and subtraction games for vampire and fire resist. Maybe that is not the case at all.