If you have a fire giant who's part frost or vice versa are they immune to both cold and fire? Obviously, they can't be immune to both and vulnerable to both at the same time.
If for some reason you have both immunity and vulnerability to something the immunity will trump it. To put it another way, 150% of zero is still zero.
Unless the effect granting vulnerability says otherwise, as a few of them do.
I would probably either have the effects cancel out or grant a minor degree of resistance to each energy type.