It is becoming clear with passage of time that Vitiate is not a living being but an entity. He was a mortal living being during in earlier times but he began to change in the aftermath of Nathema event. He maintained corporeal existence for some time but eventually began to manifest in the form of Children and Voices. He can exist as a disembodied being as well.
Seeing as he had the glow effect again when he was stabbed, and based on what he says when he gets stabbed, yeah, he's almost certainly a voice. I mean, that'd be the worst way to kill him off permanently, so he has to be a voice.
Nothing you said contradicts my post whatsoever. You're simply arguing that Vitiate is more impressive, not that Sidious isn't similarly a formless entity.
Basically, this is true of pretty much all characters. Sure, unlike most, Valkorion can continue to exist independently even after the death of his mortal form. But so to can Exar Kun or Sidious or Yoda. The only real difference with him is that he can assume multiple forms at once.
Then again, his ability to do that is limited as well. When he takes up the mantle of Immortal Emperor of Zakuul, the Sith Emperor becomes silent. When he takes up the mantle of Sith Emperor, the Immortal Emperor of Zakuul grows cold and distant.
He only truly operates/functions through one form at a time, and leaves his alternate identities on standby.
Senya supports it strongly, Marr supports it, and evidence in general supports it.
For centuries pre-SWTOR, presumably during the forging of Zakuul, Vitiate went silent in the Empire. When he returned to commanding the Empire, Senya noted that Valkorion suddenly and unexpectedly became cold and distant, and never spoke to her or her children.
You do realize that Senya is talking about Valkorion just ignoring Arcann and Thexan, yeah? She's talking about him being a shit parent, not literally disappearing.
All she says is immediately after they were born he wanted nothing to do with them and he started grooming Vaylin to be insane. If he legit wasn't there, I doubt they would choose him over her.