Piett was a lowly Captain on Vader's ship, The Executor, serving under Admiral Ozzel. Fortunately for Piett, Ozzel royally fucked up and Vader was like, "Ish don't think so." and BLAMMO, Ozzel = deceased. Vader then gave the job to Piett, which offered an awesome 401k and much better benefits. Also, The Executor was no pussy Star Destroyer. Nope, The Executor was a massive, mind fucking, SUPER Star Destroyer. To gauge the size of a Super Star Destroyer all you need to do is look at North America. Then multiply North America's size by roughly 70 trillion and there you go: Super Star Destroyer.
So how exactly did Piett get through Return of the Jedi without suffering a force choke from Vader? Well, I've been thinking long and hard about this, and the only answer I can come up with is that Piett had a bootleg copy of Vader slaughtering the younglings from Revenge of the Sith. Imagine if that shit leaked on Galactic Youtube; Darth Vader murders children, chokes out his pregnant wife, and has asthma. Why was anyone scared of him again? Well Piett sure wasn't. Piett was scared of no man. It is, however, well known that he was very pussy whipped by his wife. What's not so well known? His wife was Yarna D'Al'Gargan.
Piett was fighting at the Battle of Endor (more likely he was on on the phone asking his wife what he needed to pick up at the grocery store) when an A-Wing fighter flew into his head. Seriously, what a fucking way to go.