Quote Originally Posted by factotum View Post
I'm pretty sure that's exactly the reason for that. It was showing that Londo's corruption ran so deep that he could no longer perceive the Vorlon--or else he saw something horrible that he didn't want to admit to.
My understanding of the intent (don't have a source, don't recall where I got this) was that it was because Londo has no faith, no true belief. That the way Kosh's power works is that it taps into something you truly believe in. So Dr. Franklin, a follower of Foundationism, saw Kosh as a blend of the species he was familiar with due to his universal beliefs, and his belief that god, whom Kosh's ability is supposed to make him resemble, if only as an angelic servant thereof, is too big for words, too universal.

But Londo believes in nothing. He doesn't even really believe in himself, and at that point, he can't even be said to really believe in the Centauri Empire or its people. So he saw nothing.