borcu,
While marine fish do absorb water from the aquarium, vitamins just added to the water will be of little benefit because the amount would have to be very high, and remember that the aquarium is filled with bacteria (good and bad) that will be able to access those vitamins better and faster than the fish can.
Even a food soak is of undetermined benefit because you cannot know how much of the vitamins are even being swallowed by the fish and how much is being rinsed off into the water.
The best way to supplement vitamins is through them being bound to a food item; good quality flakes, pellets or gel diet. Avoid the temptation to think of vitamins as being any sort of a substitute for proper aquarium care; they are not medications, fat soluble vitamins can be toxic if overdosed and water soluble ones are just wasted if overdosed. The two critical vitamins for marine aquarium fish (that are often lacking in natural foods) are thiamin and vitamin E.
Jay Hemdal