Hi,
The URL would point to GET_EXPANDED_ENTITY and not to GET_EXPANDED_ENTITYSET. Thus make sure you implement the correct method.
Thus in get_expanded_entity you have to pass ls_vendor_details to the response and not lt_vendor_details. So please go ahead with excluded data = 'ITEMDATA/ACCOUNT_DATA' and copy_data_to_ref( EXPORTING is_data = ls_vendor_details CHANGING cr_entity = er_entity ).
Please check and update us.You can also check: Multi-Level Expansion with GET_EXPANDED_ENTITYSET
Regards,
Ekansh