Well I don't think that means Vader is actually on Sidious's tier, it's more of a durability/willpower feat that can still work if Vader is doing it to a being more powerful than himself.
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Since when does the Star Forge amp your Force strength?
From the video-game it was presented that the Star Forge was a tool, engineered by the Rakata, that was feeding on the dark side to create battle droids(hence, why Revan never wanted to use it). And also it had the power to heal darksiders, as the Star Forge did Bastila Shan when she was fighting Revan.
__________________ RealistRacism: "Sheevites, much like the Banites, were meant to increase in power with each member. From Lightsnake to Gideon to Azronger, this was supposed to be the case. However, knowledge must've been lost in some kind of Gravid-like incident, as Az turned out to be a mid-tier debater with a sub-par track record, sh!itting all over Tempest's legacy. Sad."
__________________ RealistRacism: "Sheevites, much like the Banites, were meant to increase in power with each member. From Lightsnake to Gideon to Azronger, this was supposed to be the case. However, knowledge must've been lost in some kind of Gravid-like incident, as Az turned out to be a mid-tier debater with a sub-par track record, sh!itting all over Tempest's legacy. Sad."
Plagueis and Sidious fundamentally shifted the balance of the Force*
Anyways it is not a testimony of his Force strength because it was not a sudden win. And the Force wasn't really bested/outbalanced by Plagueis: those were only his thoughts because the Force didn't respond back at him by killing him and Sidious, but it did. The Muun kept playing with it; and the Force was like: OK, bi*ch, taste a bit from your own medicine.
__________________ RealistRacism: "Sheevites, much like the Banites, were meant to increase in power with each member. From Lightsnake to Gideon to Azronger, this was supposed to be the case. However, knowledge must've been lost in some kind of Gravid-like incident, as Az turned out to be a mid-tier debater with a sub-par track record, sh!itting all over Tempest's legacy. Sad."
__________________ RealistRacism: "Sheevites, much like the Banites, were meant to increase in power with each member. From Lightsnake to Gideon to Azronger, this was supposed to be the case. However, knowledge must've been lost in some kind of Gravid-like incident, as Az turned out to be a mid-tier debater with a sub-par track record, sh!itting all over Tempest's legacy. Sad."
__________________ "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."