Edward:
Not sure I follow what you are after.
Are you selecting a Vendor on the Header section?
And if so, is it that you want to add Items, but not have the Items flagged as being purchased from the same Vendor on the Header screen?
In PO, I can enter a new Purchase Order, select a Vendor in the Header section, and in the Detail section, add Items. When the Items are selected, I can indicate a different Vendor by simply changing the Vendor on the screen.
Is this your question?
_________________________
Samuel Kopstick
S Kopstick & Associates Inc
Toronto, ON