Hi Mehraj,
Please change the code to
IF xvbak-vdatu IS NOT INITIAL.
IF xvbak-vdatu <> xvbkd-prsdt.
xvbkd-prsdt = xvbak-vdatu.
modify xvbkd index sy-tabix.
ENDIF.
ENDIF.
and see if it works. Also please check if you already have any other custom code which is undoing your change.
Regards,
Shashi