We buy a product from the same vendor under 2 different vendor part numbers, depending what is in stock. They are slightly different but interchangeable for our purposes. For products we buy from different vendors I just put the vendor I want on top in the purchase tab of the product page and the PO is generated with that vendor and number.  In this case it always picks the first one that was entered, not the one on top.  I want to buy the one ending in 604 but it always puts the 603 one on the PO. 

It does not seem to matter which one of these I choose:

Is there any way to get this to work without having to set start and end dates? TIA.