In Season 3's "Point of No Return", Lady Morella gives this prophecy:
Lady Morella: [Turning to Londo] You will be emperor. That part of your destiny cannot be avoided.
Londo: I see.
Lady Morella: [Turning to Vir] You will also be emperor. [Vir bursts into laughter] Why are you laughing?
Vir: I thought you were joking.
Lady Morella: We do not joke in the face of prophecy, Vir
Londo: Lady Morella, please. We cannot both be emperor.
Lady Morella: Correct. One of you will become emperor after the other is dead. That is all we see and all we wish to see.
There seems to be ambiguity in her prophecy saying that "one will be emperor before the other," which suggests that there is a (rather unlikely) chance that Vir might become emperor before Londo.
After Cartagia's death in Season 4, Londo is declared to be the next emperor by the Centaurum. And they had a number of reasons to, given what he had done:
- Served as the Centauri ambassador to Babylon 5
- Helped the Centauri defeat the Narn (with the help of the Shadows)
- Rid Centauri Prime of Shadow influence (and ostensibly from the Vorlon planetkiller)
- After Emperor Cartagia's death, set the Centauri policy to leave Narn
- Served as Prime Minister
- Became one of the founding members of the newly formed Interstellar Alliance
Contrast this with Vir, whom the nobility seem to be vaguely aware of, since he was the ambassador to Minbar (which ended in disgrace). There's no indication that Vir was ever seriously considered to become emperor.
Since Lady Morella's prophecy was ambiguous about who would be emperor first, there was the possibility that Vir might have been emperor before Londo. How the heck might this have been a possibility? How could Vir have possibly become emperor before Londo?