(taken from the Order of the Phoenix, page: 519)Snape: "Occulumency, Potter. The magical defense of the mind against penetration. An obscure branch of magic , but a highly useful one."
Here, we have Snape, an extremely proficient Legilimens and Occulumens, saying that Legilimency and Occulumency are both magical abilities. Hence why Muggles (non-magic peoples) and - therein - Force users themselves would be unable to counter Legilimency.
I think by describing it as a magical defence, it doesn't necessarily mean that it requires magical ability, just that it is a defence used by magicians. Kind of like how potion making would be described as a magical art, doesn't mean that it would require magical ability.
Sidious, despite his natural affinity for deception and secrecy and his immense will, wouldn't be able to counter Legilimency - especially from Voldemort. He was able to fool the Jedi Council because he was a master of a Force based technique that shielded his presence from them - as well as simply being cleverer and smarter than they were.
However he was described as 'an embodiment of hate and anger', so clearly he had such strong emotions, yet was able to control them on a day to day basis as chancellor for the Republic, so this to me suggests that he was great at controlling his emotions.
Snape: "Time and space matter in magic, Potter. Eye contact is often essential to Legilimency."The keyword, of course, being "often".
Oh yes, I know. It is still seemingly an added advantage.
Voldemort, who, again, is the most proficient Legilimens that the world has ever seen, may not be bound by the same limits and dictates that affect other wizards.
Well I wasn't just talking about Voldemort in particular, more just Legilimency in general.
Voldemort could simply possess any evil Force user and force he or she to do his bidding. Or walk them off a cliff. Or impale themselves on their own weapons.
Well that would be if non HP magic users couldn't defend against Legilimency, imo they can.
However I definitely agree with you about Sidious and Vader, they have many similarities.