With gear giving you 50% and 50% magic and fire resistance, I think you stick with the 50% magic resistance, and then the fire resistance is changed by your stages of vampirism. I believe it would make sense if the fire resistance 50% is additive (subtractive?) to your fire weakness. Because it is the same one piece fire resistance/weakness, then that is multiplied into the 50% magic resistance.
So in stage 1 you would have 50% magic resistance and 25% fire resistance (started at 50% and subtract the stage 1 vampirism weakness of 25%), meaning 62.5% fire resistance total (50 magic resistance times 1.25 fire resistance = 62.5 total resistance).
Stage 2 you would have 50% magic resistance and 0% fire resistance, meaning a total of 50% fire resistance. Stage 3 you would still have 50% magic resistance, and 25% fire weakness, meaning 37.5% fire resistance total. Stage 4 you still have 50% magic resistance, and 50% fire weakness, this puts you at 25% fire resistance.
I haven't tested this, but this is what makes sense to me. As Adam said, wait for Dagmar to dominate.