Hi Hari,
I understand your issue, you need to be sure if the vendor is available in the company code before someone raises the SC/PO for the same.
As Sushil pointed out, amending the search help may not be the right solution. Instead you can work on BBP_DOC_CHECK_BADI to populate the error to the user if the selected vendor is not present in the company code, selected on the SC/PO item. There you can RFC to ECC system to cross check the vendor's existence in the company.
Regards,
Ravi
