Vorlons are amorphous energy beings, how was Kosh poisoned in "The Gathering?"
Remember, they do have a certain physicality about them, even in that form, and the nature of the poison was such that it would affect that kind of life form using a crystalline base (note in the pilot the screen reads analyzing crystalline structure, and you filter light or refract or distort it using a crystalline structure).
Silicon or crystalline based lifeforms are still lifeforms, just not the same as carbon-based lifeforms (like us).
Yes, that's a Vorlon...and there was a physicality to them, shown by the fact that it could strike out and hit things. It's not a ghost or anything of that nature, it can be hurt and killed.