i think the answer is no - but is there a way w/out customization to output the "PO Vendor" column from the Sales Order line item onto the associated Invoice line item?

I doubt it. The reason it works on the SO is because they are linked transactions (I assume you are referring to Drop Ship or Special Order items?). The Customer Invoice is not linked to the Vendor PO.

That's what I figured as well.

That said, the customisation isn't too complicated, so it might be worth doing yourself (depends on your reason for not wanting customisation).