>>512826Even if the circumference is an infinitely long curve, there still exists and infinitesimally small point on it that can be measured with a vector that has both length and direction. This is the basis of vector calculus.
Also there does exist a shape were the vector that represents the radius has the exact same length all along the circle/sphere and that is the definition of a circle/sphere. Anything that doesn't meet that criteria isn't counted as a true circle/sphere.