This has been a fascinating debate, and here are my two cents: aside from boringly agreeing with everyone a bit, I think we also have to factor in yet more circumstances.
On the one hand, as far as loyalty is concerned, Vir, even if he didn't object to Londo instigating a Narn/Centauri war per se (which he clearly does), could have chosen loyalty to the Emperor/Centauri Prime over loyalty to Londo; he could, when Londo tells him to fetch Morden, and after listening to Refa's and Londo's conversation which include plans to sabotage on Refa's part and going directly against the Emperor's wishes in starting the war on Londo's, report all of this to the Emperor. He doesn't. Why not? In addition to all the reasons given, let's look at what would happen if he did. Say Vir, instead of fetching Morden for Londo, goes to Turhan instead, and tells the Emperor what he just heard. If Turhan doesn't get an immediate heart attack (a risk Vir isn't aware of), his response will on the one hand prevent another Narn/Centauri war (at least for now), but it will most definitely mean the end of Refa and of Londo as far as their careers are concerned. If not their lives. We have no idea what the penalty for unsuccessful plotting is with the Centauri, but I imagine that depending on what you're plotting, and depending on who the Emperor is, you get either banishment or an urgent hint to take your own life. But okay, Turhan is a benevolent fellow, so let's say the most Vir has to fear for Londo is removal from his post as ambassador and banishment from Centauri Prime.
Not the worst price to pay to prevent a war, surely? Well, yes. Except - Vir has to fear one more thing. That such an action would mean the end of his relationship with Londo. He has seen Londo forgive Adira her betrayal, but that was a different case; Londo loved Adira, and Vir with his self worth issues probably doesn't think Londo has more than some general good will towards him. (See a few episodes later his expectation Londo will be glad to have another aide and his surprise this isn't so.) And Londo, as far as we know, is the only person Vir has as a friend at this point. The only person who gives a damn about whether Vir lives or dies, no matter how vaguely Vir suspects this regard is. And this affection would be gone from Vir's life.
In addition to all the other reasons, I think this, too, is what makes Vir decide to abet Londo against his better knowledge.
Re: Regarding Vir's choice to abet Londo -
On the one hand, as far as loyalty is concerned, Vir, even if he didn't object to Londo instigating a Narn/Centauri war per se (which he clearly does), could have chosen loyalty to the Emperor/Centauri Prime over loyalty to Londo; he could, when Londo tells him to fetch Morden, and after listening to Refa's and Londo's conversation which include plans to sabotage on Refa's part and going directly against the Emperor's wishes in starting the war on Londo's, report all of this to the Emperor. He doesn't. Why not? In addition to all the reasons given, let's look at what would happen if he did. Say Vir, instead of fetching Morden for Londo, goes to Turhan instead, and tells the Emperor what he just heard. If Turhan doesn't get an immediate heart attack (a risk Vir isn't aware of), his response will on the one hand prevent another Narn/Centauri war (at least for now), but it will most definitely mean the end of Refa and of Londo as far as their careers are concerned. If not their lives. We have no idea what the penalty for unsuccessful plotting is with the Centauri, but I imagine that depending on what you're plotting, and depending on who the Emperor is, you get either banishment or an urgent hint to take your own life. But okay, Turhan is a benevolent fellow, so let's say the most Vir has to fear for Londo is removal from his post as ambassador and banishment from Centauri Prime.
Not the worst price to pay to prevent a war, surely? Well, yes. Except - Vir has to fear one more thing. That such an action would mean the end of his relationship with Londo. He has seen Londo forgive Adira her betrayal, but that was a different case; Londo loved Adira, and Vir with his self worth issues probably doesn't think Londo has more than some general good will towards him. (See a few episodes later his expectation Londo will be glad to have another aide and his surprise this isn't so.) And Londo, as far as we know, is the only person Vir has as a friend at this point. The only person who gives a damn about whether Vir lives or dies, no matter how vaguely Vir suspects this regard is. And this affection would be gone from Vir's life.
In addition to all the other reasons, I think this, too, is what makes Vir decide to abet Londo against his better knowledge.