I don't know the answer about vitamin C, but I swear by Cold Ease lozenges.
If you start sucking on them when you first start feeling cold symptoms coming on they will usually nip a cold in the bud right away, or if you wait before you start taking them they will lessen the severity of the symptoms and reduce the time you have the cold.
When I used to get cold I would feel lousy for a week. Last January I caught a cold while on vacation (I think from the plane) and I sucked on Cold Ease and only felt bad for two days.
Just last week I woke up one morning with a scratchy throat and stuffy nose, started sucking Cold Ease right away (a couple in the morning, a couple in the afternoon, a couple in the evening) and the next morning I woke up the symptoms were gone.
Get some, try them, they work!!! They have zinc in them, I guess that's the ingredient that works on the cold.