I can reproduce your problem and will explained it to the programmers. The vendor account pick root (F11 on the vendor ledger details ) should be respected if you post to the vendor account everywhere in NV2.
To change the pick root for the vendor, press F11 in the item field > F3 on Pick Root and select a different expense account. Note, I did try this and received a simular bug error.
I’ll let you know when this is fixed.