vendor code and tax code requirment

Hi All,

We are having 2 vendors and 4 tax codes and all are same country.

Ex: Vendor--12345-Tax code-P1

                    6789------Tax code-P2

At the time of creation of the purchase order system should allow tax code P1 for vendor 12345 only and if user is trying to enter other than P2 system should give an error message.

How to full fill this requirement.

Thanks

Ram