As stated by others, Vorlons do have some physical presence, and are not just energy. This can be seen when Ulkesh actively hits things in "Falling Towards Apotheosis" in season 4. It doesn't. The point is that Kosh voluntarily opened the suit in a way he ordinarily wouldn't. How can he open the suit safely if he requires all that wacky atmosphere in his quarters? Because the wacky atmosphere is a lie, a cover to make him seem more mysterious. We see later that he's perfectly capable of existing in standard atmosphere, and probably a vacuum too. And who would be most likely to know that combination of substances? The Shadows, of course. The Minbari assassin was one of the Wind Swords, the renegade Minbari who refused to stand down after the Battle of the Line, and instead went off on their own (to reappear in "Points of Departure"). They fell in with the Shadows, whether knowingly or not, and the Shadows used this one to try to kill Kosh. (These Minbari are also the same ones that had been harbouring Deathwalker ever since the Dilgar War.) Ah, but the Vorlons know that only the Shadows could possibly have a chance of poisoning them in this way. But they haven't been told about any changeling net. All they've been told is that Sinclair is responsible. That's why they want Sinclair handed over to them - to check him for Shadow influence. Easiest to do that on their home planet. Doesn't mean anyone else would ever be welcome there - they just have good reason to take Sinclair there. I guess the more hardline Vorlons simply think that the possible future importance of Sinclair's role in things is less important than the possibility he might have already been compromised by Shadows. Remember, it's usually only Kosh who thinks the younger races have much of importance to contribute. The rest of them think of us as just cannon fodder in their ideological war. Kosh fell for it because he's the more sentimental, friendly type of Vorlon. He knows Sinclair of old - they fought the first war together back when Sinclair was Valen. So he opened up his encounter suit to greet his old friend as himself. How did the Minbari assassin know he would do that, giving him the opportunity to poison him? Because the Minbari was working with the Shadows, who also knew who Sinclair was, and what Kosh would likely do. Kosh let his sentimentality almost get him killed - likely why he went so cold for a while afterwards. I'm pretty sure JMS has said that he never really intended that to be the nature of the Vorlons. He put that in there purely to sell the show to the suits at WB. He (probably not unreasonably) assumed that they wouldn't get his actual concept for the Vorlons - energy beings who are the basis for Angels and other religious figures in the mythology of countless worlds - and would have fought him if he tried to put that in there. So he pretended that it was a much simpler concept - scary monsters oooh! - just to get it past them and on the screen, after which he could do what he wanted. Also I think he said that they specifically asked him, "Ooh, what's in the encounter suit?" and he didn't want to let a pretty major spoiler out right off the bat, so he lied. .