I don't know what's in vita-chem, but Selcon is what many people here seem to use. I bought some recently because I've never heard anything other than good things about it, but it's been a very short time so I can't really say whether it's doing my fish much good or not -- I'm working on improving the condition of a (previously very sick) tang I adopted a while back; she's still got a touch of HLLE despite a varied diet, and I'm hoping the Selcon will give her the extra nutrition she needs to fully heal.
Selcon is mainly composed of HUFAs (highly unsaturated fatty acids); it's not really a typical vitamin supplement from what I understand. It does contain vitamins C and B12, but I think other vitamin supplements provide a wider array of vitamins. HUFAs are very important, and it certainly can't hurt to feed Selcon-enriched foods, but maybe someone can chime in here and let us know if it's a good idea to add another vitamin supplement as well?
I also agree with demonsp - there's no substitute for a varied diet. Just like us, fish do best when they get their vitamins from their food, rather than out of a bottle.