Originally, Sion is meant to nearly kill the Exile on M4-78, her party and Traya all think she died, Sion breaks his alliance with Nihilus and Nihilus kills him. Traya goes to convert Atris and Atris betrays her, cutting off Traya's hand and taking the moniker Darth Traya. The Exile returns to everybody's amazement and cleans house with everybody that is left, rescuing her party from the original Traya on Malachor V, yay the day is rescued.
In the cut-content mod, Sion doesn't die because they couldn't save enough of the content to completely rebuild the original storyline, you can still find Nihilus wrecking him on YT, they just modded it so Sion walks away.
TL;DR there were a few different ways the game went, that was how it would have gone before Obsidian cut M4-78 and revamped the story to rush it out in time for christmas, because Lucasarts wanted xmas dollarz.
My apologies, Nihilus killing Sion comes from the GenoHaradan sidequest on Nar Shaadha, Sion sees the base you are in explode so he and your party assume you died; then Traya tries to take on Atris as an apprentice and Atris kills her, becoming the new Darth Traya. At the same time this happens:
Unfortunately that ending to the game was too incomplete to restore, so the above was modded to make Sion walk away instead of die.
In all seriousness, Vader already learned how to defend against drain while he was still Anakin. I suppose it's never been proven that Caedus did, but I would be incredibly surprised if someone who was so obsessed with learning about esoteric Force techniques never bothered to find out about the power. It's not like Force drain is some sort of bizarre esoteric art that nobody knows about - Luke recognized when Krayt was using it on him and Abeloth, for example, and yet he didn't really die or get affected that much beyond being somewhat injured. And quite frankly, Krayt being able to drain Abeloth enough for it to affect her is a vastly superior feat to killing three featless Jedi masters. If Force drain were some sort of insta-kill move, Krayt would just use it on everyone, and Talzin could have just insta-killed Sidious.
I know Traya claims that her drain has "no defense", but she's a weakling and, quite frankly, is a pathological liar.
Sion is fodder; just cut his head off or disintegrate him.
Beyond those esoteric hacks, Traya and Sion, both of whom were combatively inferior to the Jedi Exile that got trashed by Nyriss, are obviously nothing next to Vader and Caedus.