How powerful was Vader truly?

"How about mind tricks? If Sith could use them as long as they were for malicious ends, there were plenty of instances in the films when they should have done so."

The fact that we've not seen the power used doesn't mean it was never used. Wasn't this your argument to my assertion that Vader doesn't use the Force when defeating Palpatine? (or was that just playing Devil's advocate? )

Besides, the novelization makes it clear that Palpatine is the main controlling force of the Empire (his death makes the run on the DSII possible),

He'd known this all along, in his heart of hearts-but relearned it every day: from traitorous lieutenants who betrayed their superiors for favors; from weak-principled functionaries who gave him the secrets of lords, and sadistic gangsters, and power-hungry politicians. No one was immune, they all craved the dark energy at their core. The Emperor had simply recognized this truth, and utilized it-for his own aggrandizement, of course.

For his soul was the black center of the Empire.

---

For the first time, the Death Star rocked. The collision with the exploding Destroyer was only the beginning, leading to various systems breakdowns, which led to reactor meltdowns, which led to personnel panic, abandonment of posts, further malfunctions, and general chaos.

Smoke was everywhere, substantial rumblings came from all directions at once, people were running and shouting. Electrical fires, steam explosions, cabin depressurizations, disruption of chain-of-command. Added to this, the continued bombardments by Rebel cruisers-smelling fear in the enemy-merely heightened the sense of hysteria that was already pervasive.

For the Emperor was dead. The central, powerful evil that had been the cohesive force to the Empire was gone; and when the dark side was this diffused, this nondirected-this was simply where it led.

Confusion.

Desperation.

Damp fear.[/i][/b] - ROTJ novelization[/blockquote]and Dooku hints at Sidious controlling the Senate. I don't doubt we'll see [i]some[/i] Sith mind control in Ep.III, though it won't be on Anakin.[blockquote][i][b]"Of his own free will," the Emperor assured him. It must be of his own free will, else all was lost. A spirit could not be coerced into corruption, it had to be seduced. It had to participate actively. It had to crave. Luke Skywalker knew these things, and still he circled the black fire, like a cat. Destinies could never be read with absolute certainty-but Skywalker would come, that was clear. "I have foreseen it. His compassion for you will be his undoing." Compassion had always been the weak belly of the Jedi, and forever would be. It was the ultimate vulnerability. The Emperor had none. "The boy will come to you, and you will then bring him before me."[/i][/b] - ROTJ novelization[/blockquote]This being said, I would argue that Palpatine uses such mind control as little as possible, both to reduce the chance of having his use detected, and also for the delicious irony of watching everyone else help him dismantle the Republic while believing their actions to be supporting it. I seriously doubt that the Sith are [i]incapable[/i] of mind tricks, since it would be the [i]first[/i] known power to [i]not[/i] be shared amongst all Force users. There's simply a reason for it [i]not[/i] being used by the Sith - though I would admit that Vader using the interrogation droid on Leia is suspect. Perhaps this is part of the "over-reliance on technology" theme that is shown hampering the Jedi in the PT, and becoming a parallel for the Sith in the OT. It wouldn't be the first time Lucas has retroactively added a theme to the OT by introducing it in the PT. See: "Rule of Two".

I swear there was a line in the ANH novel about Tarkin laughing at Vader's use of such devices, but as such, I cannot find it right now. Vader's "torture device" in ESB comes to mind as well, but its use is understandable there.