Basically, subtract the angle of vectorA from the angle of vectorB.
This will give the angle vectorA needs to be rotated by in order to match vectorB.
There are actually 2 delta angles between vectors, though, so if you want the smallest one, you'll need to check if the delta is > 180 degrees.
This will give the angle vectorA needs to be rotated by in order to match vectorB.
There are actually 2 delta angles between vectors, though, so if you want the smallest one, you'll need to check if the delta is > 180 degrees.