No matter what Grandma says, you cannot catch a cold simply by becoming chilled, or wet. In fact, being stressed has much more bearing on your general susceptibility to colds than does temperature or wetness. And more: A cold is not necessarily a sign of a weakened immune system. Once the virus is up your nose, it's up your nose.
Drinking milk does not increase mucus output. Milk and mucus only look alike.
Loading up on Vitamin C may reduce some cold symptoms, but studies are inconclusive thus far as to any long-term effects, and too much Vitamin C is not good for you.
Inhaling steam relieves some cold symptoms, but it does not kill the virus that started the cold.
The efficacy of feeding a cold and starving a fever is not based in scientific fact.
Despite what you think you may have read, there is no vaccine for the common cold, at least not yet.