Did you edit the ServicesSolitudeRadiantRaiment faction's Vendor Container to point to your new container?
Nope. I didn't know that had to be done. In Oblivion we never needed to edit factions in order to use a new container. But this method would also make the mod incompatible with any mod that also edited that faction, so that's problematic.
A quick note: merchants can only have one vendor container at a time. They will not sell anything kept in any other container unless it is their inventory.
Again, I don't know why Bethesda did it this way. They had a perfectly good system in Oblivion. Obviously this is to accommodate some other mechanic of the game with which I'm not familiar.
...and you will make your mod incompatible with every single mod that wants to edit that merchant. Quite an issue for a mod that just wants to add a few robes. You might want to try another method if that is what you were attempting.
You sound as if you think I want to purposely make my mod incompatible or that I don't care if it is. In fact, wanting to alleviate any conflicts is precisely why I chose to add a new chest and set ownership as the vendors' faction. If I didn't know any better, or if I wanted to just use this mod for myself, I'd add the items to the existing/vanilla container and save myself a lot of headaches.
So..... I guess I'll just have to make these robes craftable which goes against every fiber of my belief system for how this game should work. Using a forge to make leather armor already pushes my suspension of disbelief to the breaking point, but a cloth robe? Ridiculous. I suppose I could add a brand new vendor but then I'd have to deal with Radiant AI and all that crap. As you said, I just wanted to add a few robes.
Thank you Thomas Kaira. I'll find a way.