If I understand your question correctly, they are two entirely different things:
The "circling to land after an instrument approach" is a procedure which can take place within a specified radius of the airfield, (usually within the normal visual circuit pattern), and involves an IFR aircraft making an instument approach to one runway then circling to land on another.
The height of obstacles within that area is taken into account when the visual manouvering height for the procedure is calculated.
If the cloud base is below the published visual manouvering height for the aircraft category at the airfield, then circling to land is not permitted.
If you are thinking of "an approach maintaining VMC", that went out the books many years ago, and is now known as a "Visual Approach".
A visual approach is normally conducted when an a/c flying an IFR approach to a particular runway, wishes to continue visually - usually cutting the corner onto final approach - but still to land on that runway.
The aircraft is still treated as IFR and the clearance to conduct the approach is only given by ATC if the weather minima is suitable and traffic levels permit.
Hope that helps.