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
If a creature has vulnerability and resistance to the same type of damage, they both apply. Subtract the smaller value from the larger one and apply the result.
Your example is correct. If you have resist 10 and vulnerable 5 it will count as resist 5.