I'm a bit with The Unknown Guru on this, but only as far as to say it depends on your personal interpretation of morality and what constitutes as good and evil.

As someone with what you would call a "flexible sense of morality" I can't say Vayne is really evil, despite allowing action that resulted in the death of many; its difficult to argue against the fact that the world is a better place because of him.(SPOILER) He masterminded the defeat of the Occurians and left the political situation of Archades in a state that would allow the moderate and peace loving Larsa to do as he pleases unmolested by the more hard line members of the Ministry of Law and the power hungry Senate.

I could only imagine how his actions may have inadvertently caused great political reform in Rozzario as well. Vayne's actions saved Archades and to some extent the world, regardless of whether his intention was to do so. This is something you really can't say about any other antagonist in the series.

Personally though, this does bring me up to one of the few story issues I did have about Vayne. Playing through the title, I really felt as if Vayne purposely did everything for the benefit of Archades and Larsa. His actions and his methods always struck me as an act. Like he purposely chose to play the role of the megalomaniac in order to set up Larsa as the new ruler. He removes his political enemies; and despite several attempts to thwart his plans, Vayne never seems to try and stop Larsa's movements or even kill him considering Larsa is his only remaining political rival. We know Vayne has it in him to do it so why doesn't he?

Larsa is definetly headstrong and intelligent but its obvious his naivety would always allow his political enemies to get the better of him, whereas Vayne is already a veteran of the political system and has the will to do what he knows his innocent brother could not lest he become just like himself. I really feel Vayne tried not only to protect Larsa's innocence and good nature but also to nurture his sense of justice (it should be noted that the Solidor family has deep ties with the Ministry of Law) and conviction by playing the role of the tyrant villain.

I really felt the writer intended for Vayne to be a tragic hero, but something happened in the production (like the mastermind of the project quitting) and the writer decided to write him as the atypical JRPG villain during the last battle. Its this last battle that smurfs up my reasoning cause Vayne basically does become a power hungry "god complex" villain with characteristics that are completely unfamilair of anything we've seen. We're talking smooth and calculating villain suddenly becoming like Kefka or Kuja literally between a cutscene and there is an inconsistency with the rest of the script concerning this. Throw in the fact that the whole final dungeon and last battle comes off really forced and unpolished. The whole thing reeks of a rush job in my opinion.

At least this is just my original view of him. I need to replay the game eventually to freshen my memory. Overall, I don't feel Vayne is evil but I can't say he's good either. Its why I have a difficult time saying he's a villain, he's more like an antagonist.