When having one and the same company both as Vendor and BU in the dataset, is it possible to restrict recognition of BU if Vendor has already been found?
No, unfortunately, it is not possible when using out-of-the-box options. To give some details - it is the "Vendors" dataset that is limited by the "BU" dataset, not the opposite. I.e. we search for BU first, and, after it's matched, we look for Vendor with corresponding BU Id (if they are linked). This is by design and, as I mentioned, cannot be changed without customized solutions.