Hi Susan
Maybe there's a setting that turns this off, but my understanding is that if you try to enter an item on a PO or a Quick Receipt, and the vendor is not already a primary or secondary vendor for that item, you will get a Vendor Conflict pop-up box. By clicking on "OK As Is", it will NOT override existing primary or secondary vendors.
Are you not getting that box?
"OK As Is" is the default, so you can also just hit the enter key to continue on.
Steve