Darth Vectivus Speculation

As per his force abilities, it is reasonable to assume he would develop a high level of competence with force persuasion given his occupation as a business man. Given his love of baby animals (particularly Banthas) he likely had some skill in animal friendship, which furthers my theory of his skill in force persuasion. His signature ability however is force phantom, which featured prominently in the first half of the LOTF series so there's not much to discuss here given that we already know he is capable of this. This power does suggest that he is skilled at drawing power from external sources. This suggests a possible affinity for tutaminus. The connection he forms with other life forms to create these phantoms is interesting as well. This only furthers my theory of his force derived persuasive abilities. The drawing of power from external sources and the connection he forms with other life forms heavily suggests he may have some skill in force drain especially considering Bane likely passed down the teachings through his line. Vectivus would also definitely experiment with abilities other than force phantom given the powerful nexus he has to work with. This may suggest that he dabbled in sith sorcery, though he likely didn't do anything with sith sorcery worthy enough for it to pop up in his holocron. His phantom created by Lumiya possessed the ability to levitate, though it is unknown whether or not the real Vectivus was actually capable of this. I think it's actually quite plausible. Given the gravity free environment he had to start off with before trying it out with actual gravity and the nexus he had to work with it's highly possible he was capable of levitation.

As per his fighting style, I'd assume it would be Niman or Djem So. He was described as being both fat and muscular, which suggests that he has a lot of weight to put behind his blows. On the other hand, his obesity and focus on other things (such as: business, phantoms, the nexus he lives on, baby banthas etc.) suggest a relaxed focus on lightsaber skills and practice, the type of person Niman is made for. Given the elegant nature of his lightsaber, Makashi may be hinted at as his style, though I feel this is less likely than the other two.

His armament is actually shown in Betrayal. He carried a red-bladed lightsaber, the silver hilt of which was decorated with polished black stones that were shaped like diamonds. It's unknown whether or not he possessed any other weapons, but I don't see this as very likely considering his hinted at relaxed focus on martial combat, as well as the fact that weapons other than a lightsaber aren't the norm for Sith.

As per his master's death I'd think that Vectivus killed him. To figure this out I reread his quote about ruthlessness and strength, "Strength that is never touched by ruthlessness is touchingly irresponsible." Given his views on ruthlessness he was almost certainly ruthless enough to kill his master. Considering how strength is mentioned in this quote, it's likely that he killed his master in a fight, though I'm not 100% sure.

As per his role in the Grand Plan I'd assume it's pretty obvious. As a businessman his role almost certainly included amassing a fortune for future Banite Sith to use to bring down the Republic. He also likely studied and discovered some new powers, though how many of these actually survived Gravid's twerking if any remains unclear.