Vitiate doesn't die of old age. Truly immortal? Not quite. After his defeat at the Dark Temple, in fact, his own vulnerabilities are thrown into stark relief, and the Empire he so painstakingly built begins to fall apart, with Malgus and the Dread Masters rebelling, and (high hopes mode on) the true Old Sith showing their ugly faces again...
No indication that Darth Nox is immortal, even with his powers. One who knows what he is up against could succeed in killing him (say, the Barsen'thor, who knows how to deal with ghosts and their proxies, or powerful Sith that use ghosts for power too, by extension).
Revan never dying has not been proven fact yet, and (high hopes mode on again) these expectations will be debunked soon. Anyhow, the only thing that kept him alive beyond his time was Vitiate, and now Vitiate has no more interest in doing so. Ergo, his death is soon to come, even if he resurfaces.
And Scourge... well, I don't know to what extent he remains immortal after his rebellion (never played through JK story to learn more about him, so...). Besides, despite his undying status (different from true immortality, that doesn't die and can't be killed), his power isn't the greatest, and he is also committed to the defeat of Vitiate.
I said "immortal" meaning they won't die of old age, not "invincible" meaning they can't be killed. Big difference. Vitiate is proven immortal in the Revan book. Nox is more "invincible" than "immortal". Because he has bound those spirits to his own, he is essentially "dead" as he is now tied to the "spirit world", so if he is already "dead" then he cannot be killed.
The reason I mentioned Revan is because no matter what he lives through everything. He will be brought back over and over and survive every encounter, becoming as op as Luke in the EU.
As for Scourge, Vitiate used some of his power to make him immortal the same way he did, although because it was a cheap version of the ritual he used on himself, Scourge is in constant pain. And if you read up on Scourge, he is in fact pretty powerful, even without being immortal.