To cover a possible vitamin deficiency, I would just get a bottle of Poultry Cell or Nutri-Drench, as those are two that are a household name in many a chicken owner's box of goodies.
What I'd do is add it to their water twice a week. On a different day, give them probiotic (not electrolyte) water. Do that for a couple of weeks, then stop with the vitamins. If it was a deficiency for that one chick, it should show an improvement.
By the way, next time you need feed, I'd go with Kalmbach's Flock Maker, 20% protein. We use that in crumble form for all chicks and chickens (silkies) here. We have a dish of oyster shell for the hens as that's higher protein, but lower calcium.
