This code:
LOOP AT it_vbap1.
READ TABLE it_vbap1 INTO it_vbap2 WITH KEY vbeln = it_vbak-vbeln.
DESCRIBE TABLE it_vbap2.
ENDLOOP.
.. is not very clear because you are using header lines which (unfortunately) have the exact same names as the internal tables. This leads to errors like the one you are describing.
I would do it this way:
LOOP AT it_vbap1 into wa_vbap1.
READ TABLE it_vbap1 INTO wa_vbap2 WITH KEY vbeln = wa_vbak-vbeln.
ENDLOOP.
Actually, even then it doesn't make much sense! What exactly are you trying to do?
cheers
Paul