Organic molecules derived from vitamins that act as are called

Coenzymes

Coenzymes are small organic molecules that, themselves, do not function as catalysts (i.e. enzymes) but aid the latter in carrying out their functions. Enzymes are biological catalysts, which means that they allow reactions to occur at lower activation energies. In a way, enzymes help to "speed up" chemical reactions. 

Coenzymes are mostly derived from vitamins. Examples include TPP from Vitamin B1 (thiamine), FAD from Vitamin B2 (Riboflavin), and NAD+ from Niacin.