Default tax code in PO

Hi All,

I need Default tax code in PO/NON PO Invoice with respect to vendor .

For ex i am having 2 vendors "A" and "B" for a company code INXX

        For vendor  "A" i should get tax code "SA"

        For vendor  "B"  i should get Tax code "SB"  in invoice booking with purchase order and without purchase order.

please let us know how to full fill this requirement.

Thanks and regards.