Hi
Satendra made a good point. Looping over VBAP which is normally huge will cause performance issues when you try to load and your timeout is highly likely.
I suggest you either use the standard LO extraction possibilities with 2LIS_11_VAITM and enhance this DS with a CMOD lookup. You get delta and you can do whatever manipulation you are planning on doing.
Second option you load VBAP with a generic DS and use that delta and do the remaining manipulations in BW.
hope it helps
Martin