What do you mean by “get cell center coordinates”?
The keyword to write the cell center coordinate into the vtk file is VTK_SCALAR_CC(VTK_REGION)=.true., where VTK_REGION is the index of a defined vtk region.
If you want to use the coordinates in the code, the variable is CENTER_S(IJK,C), where IJK is the cell index and C is the coordinate index, C=1 means X, C=2 means Y and C=3 means Z
Before using it i the code, make sure you have
USE geometry, ONLY: center_s
at the top of the routine so you can access it.