I've never done a vitamin soak. I'm not sure if its safe for their outer skin. You could try to force feed some fruit flies. Mix them with a teeny tiny bit of dechlorinated water and a tiny bit of vitamin and gently spoon them in. If you do, wash your hands thoroughly, rinse super well, and rinse your hands again with dechlorinated water. Use a plain, unprinted paper towel moistened with dechlorinated water and wrap it around him like a bunting, but not too tight. Then try very gently to open his mouth and spoon the mixture in carefully a little at a time. Be careful not to choke him with too much fluid at a time. A small syringe works even better.

This us not a very clear photo, but it shows how.