Anyone have any idea why the hologram vendors deal in bottlecaps? Surely this is a plot hole as the holograms reflect the dead people pre apocalyptic world. They should be dealing in Pre War money which would make sense as the SM chips & Pre War Money are the only available currencies in the DLC.

They don't. It just acts like any other vendor's screen, putting caps as default. You can still trade pre-war money with them.

They also sell pre-war money, which allows you to unload some of the stuff you've been carrying around and turn it into weightless cash to make room for more stuff.

It's more like a game limitation. Seems caps had to be the default currency for everyone (DLC land included) or else it wouldn't work.

Little annoying is all. I'm sure the vendors could have been scripted deal in other currencies.

They don't deal in caps. There's just a GUI indication that they have none.