Nothing wrong with doing a few laps with a cold (as long as it's not serious). Just take it easy, drink water and once you get tired go home so you don't run your body down.
Also, taking vitamins after you get cold symptoms usually does nothing. It's a placebo effect for most people.
You need to be healthy and taking care BEFORE you get sick. Fyi, the most common vitamin deficiencies among snowboarders are vitamin d (lack of sunlight) and omega 3 (nearly everyone is lacking enough omega 3).