I seem to loose vacuum (for the door locks) more rapidly when the car is outside on a cold day as oppose to when the car is in the garage. There's only enough vacuum to last the locking and unlocking of the doors for about 3 times post-engine shutoff.

I have just replaced the check valve (yellow) for the door locks and shutoff valve. I don't believe that there is any further vacuum leak in the system as it locks and unlocks all the doors about 8 times post-engine shutoff when it is inside the garage.