Vader vs. Caedus
Jacen is not as strong in the Fore as his uncle or grandfather, but he is pretty close. As such he would easily crush the 1/2Grandpa that Vader post-Mustafarr was.
Vader vs. Sion
Vader is not that good at Dun Moch, and Sion is kind of unkillable otherwise unless you strip his ability to use the Force to hold himself together or something. A technique that Vader even if he had the Force potential to use, did not know. Sion can't actually beat Vader in a fight though. SO its pretty much an eternal draw until someone gets bored.
Vader vs. Nihilus
You need to be a Wound in the Force or a stupidly powerful Force User to off Nihlus. Vader is not the former, but he is/was the latter. I give him a chance to beat Nihilus, not a "given" victory like it would be for say Sidious or Vitiate or Ragnos. But he has a decent shot.
Vader vs. Kun
Vader even weakened, is still probably stronger in the Force than Kun, but neither of them OVERWHELM each other in power so this comes down to who is the better duelist. They both are the best of their era. Vader can probably take more punishment but the cybernetics may slow him down a bit. On their best days I would give it to Vader since I hold their swordsmenship about level and Vader's Force Potential a bit higher even after his injuries.
Vader vs. Vitiate
Vitiate may be a tough one. Vader can kill the avatar no problem, and keep killing. But can he actually reach the true body? And even then Vitiate's Force abilities from what I could gather from TOR and "Raven" are edging on Plageius or Ragnos's level.So he may be able to overwhelm Vader with the force before they have to cross blades.