This is NOT from me, this is from Endershadow off site
In regards to Darth Nihilus's Severe Force ability, I believe he needed help from another Sith in order to sever Darth Traya's connection to the Force IIRC.
Yep here is the video regarding the downfall of Darth Traya. Someone alongside Darth Nihilus to remove her (Traya's) connection to the Force
https://www.youtube.com/watch?v=QCxcJugMe_Y
Also the Force Drain on Traya wasn't permanent.
Apparently after that the attempt to sever her connection to the Force, she can still use some of her force power from her profile back at VS Battle wiki:
"entirely drained three Jedi Masters of comparable power to Darth Sion and Pre-Malachor V Meetra Surik, the combined Force Drain/Sever of Darth Nihilus and Darth Sion failed to drain her of her power completely, finally, at the time of her death, ..."
And yes this is in regards to the thread with Darth Nihilus vs Darth Vader part.
Also Force Siphon is not a thing though since it seems to be not applied on Darth Nihilus's profile.
Anyway, the reasoning for Darth Nihilus to sever Darth Vader's connection to the Force to be questionable given this statement going back to how Traya was defeated was with help:
"Darth Traya indeed teaches the newly christened Darth Nihilus to harness his life-draining gift to radical heights—so effectively, in fact, that Nihilus saps Traya's powers in a calculated double-cross. But Traya underestimates her disciples' depravity. They turn on her and drain her Force powers."
The source of the statement came from this website:
https://comicvine.gamespot.com/prof...th-nihilus-respect-thread-updated-2017/97172/
Not only that, but the fact Traya did indeed still retain some if not all of the powers she has although weakened during the time of her exile until a certain point.