I've found that the level 4 named vendors in particular all seem to be bugged. Every time I talk to the Vault-Tec Rep (who runs my general trading post), I have the same interaction. He runs through the "look at us, here in Sanctuary. Preparrrrrrred for the future" line. Then I press talk and I say "Hey, now that you're back here in Sanctuary, how would you like a job?" despite him clearly already having a job. Then i talk to him again and he says "They didn't tell much. Vault-Tec. I was supposed to win a pack of steak knives!". Then i talk to him again and I say "what have you got to trade today?" and I can bring up the barter menu. I can barter with him, but getting to it is always this exact same process. I can't trade items with him though. I tried to give him a better gun to use and the dialog tree just doesn't come up for him. He just goes through the same few lines.
Anne Hargrave runs my clothing stand. Every time I walk up to her stand she initially says "we put on some really great plays" or something like that, and then I click 'talk' and say "let's see what kind of clothing you're selling" and I can barter. If I try to talk to her outside of business hours, she won't respond at all. I don't get a dialog tree at all.
The Scribe is assigned as my armor vendor and I can't even get him to stay at my settlement. When he is at my settlement, I can barter with him, but he won't go to the actual armor stand despite being assigned to it. As soon as I leave the settlement, he disappears. I've called him back several times with console commands, and he inevitably just disappears again.
As far as the generic settlers that I have assigned to my vendor stands are concerned, I haven't experienced any issues with them. I can't 'trade' with them (as in equip them with different clothing or guns) during business hours. If I talk to them outside of business hours I'm able to. I don't think this is really a bug, although it's possibly some poor planning. It would be nice to still have a 'trade items' option when they're working at the vendor stands, but I think the lack of the option is simply due to them assuming the generalized vendor dialog tree during business hours. Allowing items to be traded during business hours would require implementing a separate vendor dialog tree than the one that's implemented for other vendors around the Commonwealth. This may not have been too much extra work, but just giving them the generic vendor dialog tree was the simpler solution. I don't have a problem with this. I don't often trade items with my settlers in the first place.