Splay legs are *usually* apparent at hatch.. not 3 weeks out.
I've not heard of anyone hobbling a 3 week old chick and seeing improvement, before you.
My *guess* would be the vitamins are working. Your feed routine didn't sound extremely off to really cause a deficiency.. BUT some individuals for whatever genetic reason may require more support than the standard chick.
It's also possible you may be looking at a valgus or varus type or other congenital/hatching deformity.. maybe hold the chick in a manner we can better see what you're describing.
Glad you are seeing some improvement!