__________________ "Vader's pulse and breathing were machine-regulated, so they could not quicken; but something in his chest became more electric around his meetings with the Emperor; he could not say how. A feeling of fullness, of power, of dark and demon mastery -- of secret lusts, unrestrained passion, wild submission -- all these things were in Vader's heart as he neared his Emperor. These things and more."
Luke Skywalker and Darth Krayt defeated Abeloth. Krayt's Force Drain powers proved beneficial in this contest.
In this contest, both Darths Vitiate and Nihilus are masters of Force Drain powers and also happen to be world-busters. Abeloth cannot defeat this duo.
Considering the level of inconsistency of Apocalypse Krayt and how he was oddly depicted as Luke Skywalker's peer, I don't think a comparison to Nihilus and Vitiate using Legacy Krayt (who hasn't even shown an affinity for Drain) is a valid one. More importantly, she was also active in other parts of the galaxy and was not using full power against them.
In past instances, it required both the Daughter and the Son to actually beat her. In fact, it's suggested that a light sider and dark sider are required to beat Abeloth - entirely, at least.
Because anybody with a brain would realize that Abeloth blows even Reborn Krayt apart. Krayt's powerful, to be sure, but suggesting he can compete with Abeloth in the slightest is just laughable. Abeloth's apotheosis in general renders her above standard Force users, and that includes Nihilus and Vitiate, because, although their conditions/past events have given them incredible/unique power, they were nonetheless natural Force users (at birth, at least).
Abeloth might be too much for Darth Krayt to handle (solo), but he proved to be a force to be reckoned with alongside Luke Skywalker.
In-fact, I recall that Luke Skywalker managed to defeat an avatar of Abeloth single-handedly in a cave setting by using the cave's environment to his advantage.
As for Abeloth being stronger then Darths Vitiate and Nihilus, this is an assumption based on the fact that Abeloth is stronger then Luke Skywalker. However, for comparison purpose, Lord Nyax was stronger then Luke Skywalker, Mara Jade Skywalker, and Tahiri Veila (possibly combined) during the NJO era.
And he was tired of these creatures. They were weaker than he, but so stubborn. Even inventive.
Luke using special techniques or plot holes to defeat a weakened Abeloth temporarily isn't exactly the same as Vitiate and Nihilus trying to permanently kill her.
And that involved a nexus. Nor is it a legitimate comparison in the first place.
Abeloth could defeat or kill Luke Skywalker, I do no deny this. However, I am pointing out events which reveal that Abeloth isn't unstoppable in a confrontation.
As for this hypothetical confrontation, I am not sure what are the requirements of victory.