Provisioner Tokens
They appear when the event chain in that area is completed. There are 5 chain events in VB, 3 in AB, and 4 in TD.
TD is the easiest. The chains take ~30 minutes but the meta only occurs every 2 hours, so the vendors are usually up for 90 minutes.
The chains are most likely to be completed on high population maps right before the meta starts. Use:
http://dulfy.net/2015/11/09/gw2-hot-maps-timer-famme/
To determine when maps are close to being done and use lfg to taxi into a good map.
It generally takes me about an hour to get 10 tokens out of the 12 possible per day.