Hi Ritesh,
You could do that using the at new condition loop for that field, say. ebeln, if you want just want one row to be populated with vat value for each ebeln in the the table.
Just ensure, ebeln is the first field of the table it_alv and that its sorted by ebeln.
Example,
sort it_alv by ebeln.
LOOP AT it_alv ASSIGNING <fs-alv>.
at new ebeln.
READ TABLE it_vat ASSIGNING <fs-vat>
WITH KEY ebeln = <fs-alv>-ebeln
posnr = <fs-alv>-posnr
kschl = 'JVCS'.
IF sy-subrc = 0.
<fs-alv>-vat = <fs-vat>-kbetr / 10.
| <fs-alv>-vat_val = <<fs-alv>-vat * it_alv-menge. |
END IF.
endat
END LOOP.