It frustrates me that some objects in TS4 only auto-call NPCs on certain lots. I was so disappointed that City Living food stands would stand empty on park lots. 😦
Previously I was able to create mods to make it free to hire vendors, and bartenders and baristas. However, I still wished that the NPCs would auto-attend the lot because it breaks the immersion to have to keep calling them in manually. After MUCH playing around with the files, and many failed attempts, I was finally able to call some NPCs in to certain lots.
For the vendors, you can place multiple stalls on one lot but only one vendor will be auto hired. They seem to choose which stall to open at random. If you delete a stall, they will go and open another. The vendors will auto tend any type of stall though: food; tea; swag; produce.
The baristas and vendors are called to the lot in overlapping shifts, so you should see a new NPC arriving to replace the one that is leaving. The barista/vendor will be called even if you do not have a barista bar or stall on the lot.
