Why vampiric immortality is morally justified
The human world is an asset. What can humans achieve? They build things, they build their lives, then they die and return to nothing. The end result is always nothing. Humanity is the very definition of a zero sum game. A vampire is someone drawing power, drawing life, without giving anything back. If performed within the game, the sum is still the same. One side loses, the other gains. In the end everything will be balanced.
An immortal is not within the confines of the game anymore, the sum is no longer balanced out to nothing. If the vampire draws from the assets, draws from humanity, this amounts to taking from nothing, as the end result of humanity is nothing. Immortality is beyond this game, it has a higher value. It no longer returns to nothing.
If by drawing power from the zero sum game, a higher value is achieved, this is a GAIN.