The influence of in-plane field on three kinds of hard domains was studied experimentally by means of direct current bias fields reconstruction. The experiment verifies that VBLs in those three kinds of hard domains collapse gradually under the in-plane field. And the conclusion we obtained that these hard domains have the same range of the critical in-plane field ［H1ip,H2ip］ corrects the previous view that (H1ip)IIDH1ip)IDH1ip)OHB. By revealing that the breakdown of VBLs affected by in-plane field is independent of the rigidity of the hard domains, this paper provides an important clue for interpreting the mechanism of the VBLs' vanishing and a new scheme for exactly measuring the critical in-plane field under which VBLs begin to disappear.