To call virtual_copy you have to first construct the parameters. First, the two struct vir_addr parameters specifying source virtual address and the destination virtual address need to be built. Here is that struct's definition:

struct vir_addr {
  int proc_nr_e;
  int segment;
  vir_bytes offset;
};

  proc_nr_e:   endpoint value of the process
  segment:     This should be T, D, or S (0,1, or 2)
               T for Text, D for Data, S for Stack
  offset:      Byte offset from the beginning of the segment.

These must of course be set correctly in order for virtual_copy to work. Several tests are made in that function and the most likely result is an error code return value if the parameters are not set up as expected.