using onmetis to get a vertex separator

onmetis generates a file which stores a fill reducing ordering of the vertices. Is there
a way to use onmetis to get the size of the vertex separator for that ordering?

RE: There is the routine called

There is the routine called METIS_NodeNDP used by the parallel ordering in ParMetis that returns additional information (i.e., the sizes array) that can be used to get to this info. You will need to understand what the sizes[] array stores by reading the documentation of ParMetis.