MPI_Type_vector enables the definition of non-contiguous vectors with constant stride. Where might you use it? One possible scenario is in a datatype that corresponds to the rows of a Fortran matrix. In Fortran, due to column-major ordering, row data are not contiguous. They skip nrows entries for every element. Therefore the stride parameter naturally equals nrows:

! Fortran code to create vtype, an MPI row datatype for matrix A
call MPI_Type_vector(ncols,1,nrows,MPI_DOUBLE_PRECISION,vtype,ierr)
call MPI_Type_commit(vtype,ierr)
call MPI_Send(A(nrows,1),1,vtype...)

Notice that it takes two steps to create a derived datatype: a call to MPI_Type_*, followed by a call to MPI_Type_commit.