Pradeep Chintala is right!
vbak-bstnk is not long enough!
if you va01 create a SO with customer po num's length exceeding 20, you'll find the vbak-bstnk is incomplete.
and you would never find the vbak-bstnk in screens of va01, va02, va03
but you can find vbkd-bstkd.