I don't think it's really fair to say there's no way to define a temperature in a vacuum. With radiation, it seems fair to call the temperature that to which a black body would be heated if placed in the vacuum.
Outer space is very, very far from thermal equilibrium. The temperature of the interplanetary plasma is different from the temperature of the redshifted-big-bang-light (CMB) which is different from the temperature of sunlight. The temperature of a black body depends on how much of it is exposed to sunlight vs. CMB.
I'm well aware, but I was just talking about the easiest case (deep intergalactic space with the CMB and minimal other exposure to radiation). There's a lot more complication than that, as you indicated, but I still don't think it's fair to call it undefined due to the lack of matter (I'd grant calling it undefined in some areas due to the complication of the different sources of radiation and of the matter, but that's another story).
