Hi Abhijeet,
here i am showing you some scn help to use maintain bapi of vmd_ei_api class, i think you are missing some data to be uploaded along with there are methods in lsmw, bdc too for this, please go through below links carefully , i hope you will able to solve your problem.
Vendor Master Upload Program - ABAP Development - SCN Wiki
unable to vendor master record update using class vmd_ei_api