Regarding bringing the vitamins you mentioned through Customs in the US (I can't speak for Europe) it should not be a problem. Also, travel-wise, if you have a product there called "Airborne", I'd recommend it for a long trip. It is basically a gram of vitamin C in water-soluble form with some minerals and vitamins. I don't know how much difference the latter makes, but the C surely must be good for boosting immune system for those long flights.

Regards, Jim in Salt Lake