Can anyone possibly share a video or save file? We've had trouble reproducing this issue thus far. Thanks so much for the help! http://answers.ea.com/t5/The-Sims-4-Bug-Reports/INFORMATION-How-to-Send-User-Files-to-The-Sims-4-Tea...
Here is my save file.It's the 50x50 lot in Sulani that's the active family right now. I do have a fireplace. Nalani Mahi'ai always spawns as a vendor and stands around either in my lot or just outside it, but there's already a vendor tending the stand in the area. Resetting her does not fix anything, and I cannot ask her to leave. The sims that live there are celebrities, and at first I thought she was a mis-tagged fan or stan, even though neither of the sims living in there have the stan quirk. I can travel to another lot and invite her over and she'll be gone when I come home, but she inevitably respawns shortly after. Sometimes she'll come into my house uninvited, but she usually doesn't do anything except stand around.
I played around the household getting the vendor issue some more and thought to take some videos of it.
This one shows that sims labeled as vendors aren't just coming to my house, they're also following me around, even out to the middle of the ocean and just staring while I free dive.
This one was taken a few minutes later when I sent my other sim (Venessa from Get Famous) back home. I noticed someone standing idly on the beach when Venessa came out to dive at the buoy but didn't check their status, but they were still there and they were also a vendor, but this one didn't follow me into the water (perhaps because she isn't a mermaid like the other two?). Then when Venessa ran by the vendor stall, the vendor running it closed shop and followed Venessa home, stood around for a few hours while a new vendor tended the stall. When that vendor left, the stalker one went back to doing his job. It seems like something is causing them to get distracted by my sims instead of completing their jobs, which causes another vendor to spawn so they can't go do their job, and they don't just leave.
It hasn't happened since I thought to record, but I've also had vendors end up in my house. I haven't yet seen them come in so I'm not sure if they're spawning from the fire place or just walking in like the City Living apartment bug. So far I've seen them in my living/dining room, which is the room with the fireplace, or on my back deck, which you get to from the room with the fireplace, or walking out of the living/dining room into the hall. Usually my only options are to introduce myself because I don't know the sims, so it's definitely not that my sims are inviting them in. Even if my sims know them, I haven't had an ask to leave option because it's like the game doesn't recognize they're guests, presumably because they weren't interacted with before coming inside. Visiting another location on the map and then coming back will get everybody to go away, at least for a time.