Hi again.
Let me ask you this. What's inherently virtuous about being a vampire? Is it the assorted powers you get?
You mean that crappy drain life spell? Also, as previously stated I find them immoral just as I do the very concept of war and violence however I find Harkon's morals behind their trying to stop the prophecy akin to wiping out the Dragons, taking over Skyrim/Liberating Skyrim or almost anything your partake in. However as for being a Vampire, one can assume not all that many are turned willingly and from fighting them we can also say they most likely don't possess the means to cure themselves and I do find the paranoia of death a reason but arguable for the Dragonborn as he knows there is an afterlife through interactions with Daedra and other ways (spoilers so I wont post here).
We can also assume that feeding is an urge such as hunger for us humans and humans have done horrific things from hunger so factoring in the length it would take to cure yourself feeding would happen eventually and once started it could very well be difficult to stop.
I'm not saing that I'm going to try and enforce my concepts. I'm just saying that Harkon's views are too far. He's planning to subjegate all others just so that his people could survive. It is no different thatn an Argonian drowning the world or a nord covering the land in snow. Cause a vampire will die in the sun just as a nord will drown in water, or any race besides a nord will suffer in the cold.
It's actually to stop a prophecy if I remember correctly the 'tyranny of the sun' refers to a prophecy on the vampires being wiped out (E3 interviews are the source of this knowledge, I don't think it's pre-existing lore)
Though I think a vampire can be morally good or netural, for the most part they are evil. It has been shown that vampires can cure themselves in several different ways as well as the fact if they do remain a vampire they can feed off of people who would be considered bad. A good example is count Hassildor and his wife. His wife wanted to be cured and refused to feed on anyone and Hassildor only feed on criminals from the prision. From what I've seen so far I do not belive the Volihkar are like Hassildor.
I still wouldn't call it justifiable though as once again who is 'good' and 'bad' is a subjective claim.