This was addressed by JMS in an interview on the Midwinter site.
Q. Was he really poisoned, or just pretending? ("The Gathering")
JMS: No, he wasn't faking it. Understand that their appearance as a being of light is only how they want to appear; they are life forms much the same as many others, and can be poisoned if one knows the right combination of substances.
So, although the Vorlons have the capacity to turn themselves into beings of light (temporarily?), that doesn't make them functionally immortal against what Lorien describes as 'illness or injury', especially in fisticuffs against another being that is largely the same as themselves.
JMS noted that although the Vorlon appear as swirling light to us, there's also a "physicality" to them that allows them to be touched and to touch.
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.