Posting this in case it helps someone else.  (Client is Premium, if it matters).

After a lot of troubleshooting, a colleague confirmed the issue is that a vendor just alphabetically prior to the one in question had a French accented character in its VendorNo field.

01-MÉGA... - prints fine (on AP Aging / Trial Balance)

01-MESS... - won't print on the AP Aging report or Trial Balance... not even when trying to print just that one vendor using the Selection grid.  Other reports are fine.

I'm guessing it has something to do with an index not handling the french accented characters properly, and the tested fix was to renumber the MÉGA... vendor, replacing the accented character.  (The client, who is French Canadian, probably won't like this limitation, but it seemed to work).

Moral of the story: Sage 100 doesn't throw any errors with accented characters in key fields (CustomerNo, ContactCode, VendorNo...), but it is best to avoid doing that.