Ok, Renier (and those who follow looking for the same thing).
It's been interesting. After many emails, I finally got this out of the support team:
When I do a low level search of the files in the data directory, it seems that (for immediate payments) the vendor entered is not being stored - neither as the vendor name nor as the ID of the vendor. The only information stored for the transaction is what pops up in the journal. It looks like they're using the same form for data entry as the enter bill / enter payment workflow, but just ignoring some fields.
So, until the developers decide to add a "direct payments" data structure and populate it with this stuff, we are humped.