Vault-Co is going to modify Kahn's M.A.D. because it is something that has outlived it's usefulness.
If your civil defense is extensive enough then using nuclear weapons becomes a far, far less interesting option, given the guaranteed reprisals that will emerge from such a program. If a nation puts enough of it's infrastructure underground and is prepared to weather a nuclear attack, their opponent has to start worrying about an organized deterrent based on a plausible survival of major military forces on your adversary's side.
If Israel had a Swiss style civil defense program with assured survivability and hardened infrastructure, any nation with or without nuclear weapons would be stupid to attack them. Acquiring nuclear weapons would not be a trump card. It would not even be an ace in the hole. It might in fact become a liability because it justifies your enemy using them on you in the return volley.
Have a think about it. Nothing is smarter than civil defense. Nothing. You show me the average IQ of any nation on Earth, I can tell you how much they spend on civil defense per capita.