I'm not very familiar with libgdx vectors. I want to create my own joystick-controller and I want to know how can I find the coordinates of B by using Vectors. I can calculate the radius and I have the center.
Imagine the radius is one. That means every point around the circle is exactly one unit away from the center. Now what kind of vectors always have a length of one? Unit vectors of course. You can get a unit vector by normalizing a non-unit vector.
Take the vector CA (center to A). Next, normalize CA to make it a unit vector, then scale it by the radius of your circle, to get vector CB. Finally, add CB to your center coordinates to get the coordinates of B.
B = Normalize(APos - CenterPos) * Radius BPos = CenterPos + B