I really don't understand why you think that Banite Sith, a system designed to ensure that knowledge and skill is lost as the apprentice murders the master at the first sign of weakness instead of actually learning all the deep mysteries of the force, would be stronger than other types of Sith.
Apprentices only kill their masters once they've learned all they could from them. This is because they are not stupid. They want the knowledge and techniques their masters have and will wait until they've learned them all before killing said masters. The idea that they kill them before learning the mysteries of the Force is a false one.
That said, think your untrue theory through a bit. Apprentices are only at all capable of killing their masters if they are more powerful than them. Even if they didn't have all their knowledge, they still would've been more powerful than their masters, otherwise they would've failed in their attempts. If Vectivus killed his master, it's only because he was more powerful than his master, who was more powerful than
their master, who was more powerful than
their master, all the way to Bane, who under this system would've been the weakest of all the Baneite Sith regardless of whatever knowledge he had.
Banites are the reason that most of the advanced Darkside techniques have been lost outside of the occasional surviving Sith groups that decided that Bane was a moron.
Actually, in order to explain why modern Sith lack so many of the techniques more ancient Sith had, it was revealed that there was one reason and one reason alone that such was the case: Darth Gravid. He went insane trying to master both the Light Side and the Dark Side, leading to him destroying most of the Sith's knowledge so that he could teach future Sith solely according to his teachings. What little Sith lore remained was because his apprentice killed him before he could destroy it all.
Fair enough, though remember that while the Baneith Sith might have been stronger fighters, they had lost a lot of ancient Sith lore (sometimes by accident, sometimes by design). A pre-Banite even a hundred years before the Battle of Ruusan, would have a far broader education than that of Bane and his successors. In terms of quest mechanics, think of it as sacrificing a bit of Martial to pick up a lot more Lore.
Vectivus is useless for that purpose. What knowledge he has has already been collected by the Abyss Watchers. The only remaining question is how good of a fighter he is, in which case it's better for him to be Baneite rather than pre-Baneite. The only thing the choice will make now is how good of a fighter he'll be.
