To reproduce do the following

1. Have a quest reward dialogue open -- one of the ones from killing all enemies in an area or killing the boss monster of an area. (Not a story boss fight)
2. Fast travel to town (or speak to the rarely found "Fate" vendor)
3. Speak to a vendor with the dialogue still open

At this point you'll be locked out of talking to the vendor. The reason is because the window for vendors opened but your inventory is surpressed from being shown. When you then accept the quest reward the game thinks you already have the vendor window open and you'll remain locked out from the vendor.