Ah! It's getting clearer now - still not totally, but I think I'm beginning to understand...

Instead of opening the check from the bank register, I needed to open the "Bill" from the Vendor List or from the history report as per your instructions; now I have the "Expense" tab and "Items" tab. Unfortunately I see that "Marketing" is the expense account that was selected when the check was issued.

Regarding your question about an open vendor credit -- No, it is the collections report showing an AR credit on the customer account.