What? No, Vaapad doesn't feed on the force energies of the opponent. Vaapad's mechanics are almost unrelated to the force, really, at least directly--it's more about emotion.
Sith become more powerful by drawing on their rage, their inner darkness. Mace, as a Jedi, invented Vaapad to put use to his own inner darkness, channeling his negative emotions into something positive.
And just as Sith feed on the fear or anger of their others to stack on their own strength, Mace too accepts the fury of the opponent, the darkness within them, and channels it along with his own--competing the circle.
The effects of Mace's fighting philosophy are not that much different than Sith, it's simply more efficient--Probably most comparable to Scourge's affinity for gorging on the emotions of others.
Mace was at his best in RotS partially because Sidious's inner darkness was greater than the ores he faced, and also because he submerged himself more deeply into the fight--he wasn't holding back like he usually does. Really though, the difference in performance then and in his other showings is overrated. It's certainly significant, but not what is made out to be.
Windu wins, btw.