Do you have resources there to have one necropsied? One week old is young for coccidiosis, but it doesn't sound like a vitamin deficiency either. What kind of lamp are you using for heat? An incorrect bulb that is not intended for poultry can give off fumes that are deadly. Have you had any pasty butts or abnormal droppings? Coccidiosis will usually present with runny, or mucousy, or bloody droppings. Check your feed for any signs of spoilage. If it's still in the original bag, dump it all into another container and look through all of it, sometimes a bag looks good on top and has spoiled at the bottom. Any color change, green/whitish/grayish or any strong odor or clumping would mean probable spoilage. And bad feed can do this. With nothing else to go on, between vitamins and amprolium, I would do the amprolium at this point, if it happens again or if you see symptoms. Lethargy is another symptom of coccidiosis and amprolium is pretty safe, it won't hurt them if it's something else. I also might contact the breeder you got them from and explain what has happened, see what they say.
For future reference, poly vi sol without iron is what you want.