Here is the long and skinny on it.
None of us can truely tell you what to do... we have never seen the booger! All we can tell you is that splayed legs can be corrected by hobbling the chick. Vitamins and such with help give the booger nurishment, but if there is something wrong with the skeletal structure of the chick... ain't no vitamin for that!
You know how a chicken should move, you know what the chick is doing... if you think he/she can make it in the world like it is now, then try to help it... if not, then...well, you know what is next.
From what is SOUNDS to me... maybe your temps took a spike at some point (towards the end, I predict) and high temps towards the end of a incubation can lead to legs being deformed.
Also, something to concider, there is sometimes a chick that will have it's hock joint dislocated. I'm not sure if this fits your bill... but you can get a hock back in place by gently rubbing the "hip" and getting the hock joint back in place. If this is it... you would see immediate improvement... like popping a finger into place after being dislocated. Same thing, different critter.
Is it on it's back all the time? Like it can't control it's legs or like it can't get balanced and just fall?