Even without beating Vitiate the HoT is better than Revan. Pointing out her lack of experience is irrelevant since the Hero was one of if not the strongest Jedi in the Order as a padawan. The Hero has also fought more Sith than Revan has. 😉
However, the Hero did beat Vitiate and it wasn't just because he was weakened. Vitiate himself credits the Hero's prowess to her 'immense power.'
Also Vitiate did have time to recuperate, especially with a Lightsided HoT, since the Hero needs to invade Dromund Kaas and fight through the Imperial Guard to get to him. He wasn't fully recovered, but he did recover some of his strength. And as Scourge says, he was recovering swiftly. The delay in going to save your companion is enough for Vitiate to noticeably recover, and that delay was tiny.