When it comes to fish food, vitamin supplementation does far more good than any possible type of potential harm from the use of synthetic supplements. Certainly the ideal situation is to have the vast majority of the vitamin & mineral content coming directly from the raw ingredients themselves, which in a quality food will be the case. The key factor regarding vitamins is their bio-availability to the fish, so a smart manufacturer will make sure that the raw ingredients themselves contain an ample amount of all of the various vitamins & minerals known to be required by all species of fish, and will supplement beyond that to maximize the total bio-availability to the fish. Think of it as a safety net.
Certainly mega doses of certain vitamins/minerals can be harmful to a fish, just as they can be to a human, but I don't know of any scientific documentation that has ever shown synthetic vitamin/mineral supplementation in proper amounts being harmful to a fish. Quite the opposite.
Below is a link to an interesting read regarding vitamin c supplementation, and just how important it can be to the overall health of a fish.
Just keep in mind that the vast majority of today's commercial foods no longer simply use ascorbic acid (a very unstable form of vitamin c) they now use a more expensive, but much more stable form of vitamin c to ensure that sufficient amounts are still present at post extrusion levels. It is a common misconception that the majority of vitamins are destroyed during processing, which simply is not true. Some certainly, which is why it's a good idea to have a safety net in place. As long as a manufacturer takes into account that a certain percentage of some vitamins will be lost during the extrusion process, and formulates their vitamin supplementation with this in mind, adjusting the ratios accordingly, there will be no problems. Personally I would be far more concerend about a lack of some of these key nutrients in a fish food, than whether they are synthetic based or not.
http://www.dsm.com/en_US/html/dnpna/anh_aqua_vit_c_health.htm
HTH