Suppose the same creature has resist 10 lightning and vulnerable 5 lightning at the same time. I've been told that the rules are that both of them apply and neither are cumulative; I assumed that this meant that you take whichever one is larger. However, is it possible that the sum total of this would be resist 5 lightning, more or less like 10 + (-5) being equal to 5?
RC page 225, 226
Your example is correct. If you have resist 10 and vulnerable 5 it will count as resist 5.