I think of Vorlons as sporocystean entities. They are semi-corporeal, they have physical cell structure, but it is suspended in concentrated energy, so their forms are more transient than ours. Their true form is cephalopoid, but they can appear to be something else by telepathically manipulating the vision centers of younger races’ brains. It was said that Vorlons breathed a combination of methane and hydrogen sulfide, but they seem to do fine in human atmospheres, as well as the vacuum of space.