Assuming that the VOF problem involves water and air, you first create a surface at the interface using Insert->Location->Isosurface. The variable would be Water.Volume Fraction, and the value is 0.5. The isosurface can be seen in the viewer. The function calculator can be used for evaluating quantities on this surface. A curve may be created based on the intersection on the isosurface and a boundary. Use Insert->Location->Polyline. For Method, select Boundary Intersection. Select the desired boundary surface and the isosurface. The polyline can now be used as a locater in an expression, for charting, or be exported via a .csv file.