I believe you gentlemen are confusing what "Immortal" means...
"living forever; never dying or decaying." Now... I agree in the sense I don't think ANYTHING is immortal, not even gods. Even though they all fit the definition... ageless, living forever, never dying or decaying, along comes someone with more power than them, kills them... Not immortal anymore right?
Vampires are no different. They're still immortal living a life without end until someone (or thing) more powerful than them comes along and kills them. In the general sense and most popular folk lores vampires require the drinking the blood of the living in order to prolong their unholy existence, but such is not the case, not completely, in ES.
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But what you all are arguing, is the same thing... in different view points. It's like saying "Murder is wrong, but we must kill this guy because he's going to be a great villain someday".
Vampires are by definition immortal because they don't decay, they're ageless so they're never dying, and they <can> live forever if they're cunning and powerful enough to live that long. I would even put a Lich as an immortal because it's already down to bare bones, nothing left to decay lol.