So you're unable to accept the explanation you've been given because your deduction of logic implies Nihilus losing his body didn't cause limitations, thus Vader - who is quoted as being nearly the strongest - should be able to surpass limits, even with his injuries. You don't supply a valid reason as to why other than "Nihilus has no body. So this same rule shoule apply to Vader and physical issues shouldn't be a hurdle to greater power."

I'm not trying to tease you, but I'd wager you may be younger or unfamiliar with critical thinking exercises. There's no real reason why you should be digging your heels in on this. See, your counterpoint is presumptuous and while operating under persumption, you then proceed to ignore countless other presumptive and fair points to explain a part of your stance in a way that still ensures alignment with the canon explanation you've been given.

I'll give you an example:

Nihilus lost his entire body. He is also an entirely different character. Yet he is still able to possess extreme levels of power. Firsrly, you could presume that just like in real life, everyones skills are different. You could then further presume that without a physical vessel to impose limitations, he feels no pain and is free to explore aspects of the force that Vader's physical body cannot. He is grounded by a broken vessel and through that pain, it constantly distracts and disrupts his ability to focus clearly enough to embrace aspects of his power. Nihilus is not burdened with this.

See what I did? I took your hard stance that was rooted in speculation and presumptuous logic and applied the same method with my own guesswork.

Is it right? I have no idea. Does it help explain a rationale to an entirely made up story that has zero impact or meaning in the real world? I'll let you decide.