Personally I think thats a bad idea, most animal feeds do already include the recommended dosage of vitamins and adding extra can cause more harm than good (I recently asked my vet about this when one of my dogs had cancer), This also includes fish foods. If you look on most packs of food the vitamin content is listed.
I tend to soak blood worms (or other live foods) in vitamins due to the fact that they are not nutritionally brilliant. I think to soak pellets/flakes in vitamins would be over-doing it as the majority of flakes/pellets are 'complete foods' meaning all the vitamins and other nutrients have been already packed into the food.
Here in the USA we can get fish vitamins they Vita-Chem they are called. I sware that they helped my Uarus with HITH heal so fast. I sometime soak my bloodworms in the vitamines. I think they do help ill fish. if you can find fish Vitamins they can help but not nessary.
I personally wouldn't bother, a staple of a quality flake/pellet topped off with treats/greens should provide all the nutrients/vitamins your fish need.