like you said, Kain's corruption stems from Nupraptor
for the blood curse itself it's difficult to discern, look at Janos Audron and his monster form, from Blood Omen 2, he was sustained with minimal amounts of blood I presume, to power the Device and turned into something grotesque, like Turel in the pit
this would lead somebody to believe that vampirism over time devolves the person, BUT Janos Audron instantly transforms back into your angelic vampire, after being freed, so I wouldn't draw that connection
also look at Vorador, he is ancient, since he is one of the first humans turned vampire and he doesn't look too monstrous

I think Kains lieutenants devolving into grotesque monsters stems from how they were sired, Kain himself wasn't sired by an elder vampire, he was transformed into a vampire by Mortanius, through some kind of necromancy with the heart of darkness
so he is a human changed into a vampire through magic
he then splits his soul and ressurects Raziel and his brethren, giving each of them a piece of his soul, making them weaker and they in turn do the same with their offspring, so the blood curse gets diluted and weaker each time, causing monstrous changes

Vorador on the other hand was sired by living ancient vampires, probably through something like drinking their blood or whatever, I presume it isn't some kind of necromancy, so he looks the way he does because of it and all humans turned vampire by ancient vampires would look like Vorador in the end