Hi again,
Believe me I do not completely understand your problem.
Who are creating the PO?
BAdI Change is intended to be executed after user input but before saving (i.e. you change some field value and press enter, ...).
What I do not get is how a user can create a PO without modifying it. I understand the process as: user selects create PO from Portal/NWBC, fills in all data and then press save or submit. So, as soon as user fills in a value and some action is performed (check, enter, save, submit, changing value in some dropdowns fields, etc, taking into account that not all fields are able to "roundtrip" ==> check Generic Field Display on User Interface in spro), BAdI change should be executed and your custom LT should be modified. I can tell you that BAdI change is executed when you create a PO (not only when you are editing an existing one). That's for sure... Once your WD context is properly filled in (including your new LT), such context values are stored into DB via Mappers&CLL classes and PD/PDO layers.
"But once the PO is created, the system skips the badi and the vendor text still empty." ==>BAdI should be executed before saving and custom changes are also intended to be saved...
Are you creating PO from custom program, from SOCO, approved SC/Response or something similar? Cause using a normal user interaction PO creation, I think BAdI change should do exactly what you want :S. I guess that you are involuntary clearing LT in some BAdI call or so.
Just another hint, I remember some problems related to UI not getting informed about some changes been performed by coding, so sometimes "set changed by client" property has to be explicitly coded in some way, but I think it is more related to WD and this is something pretty obvious in this BAdI. Please try to also modify another field at header or item level (informing the corresponding output wa/itab) and cross fingers...
I hope you get a solution
Good luck and regards
Vicente