You must add the library file onto the vhier command line, for example:

$ cat top.v
module top;
    reg  buf_in;
    wire buf_out;
    wire a;
    inv1x i0 (.in(buf_in), .out(a      ));
    inv2x i1 (.in(a     ), .out(buf_out));
endmodule

$ cat technology.v
module inv1x (in, out);
    input  in;
    output out;
    assign out = ~in;
endmodule

module inv2x (in, out);
    input  in;
    output out;
    assign out = ~in;
endmodule
$
$ vhier top.v technology.v --modules
  inv1x
  inv2x
  top
$
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