yeah, the transformed humans are inferior and eventually devolve but there are some exceptions like Vorador, because he was converted by an ancient vampire so he received the unaltered gift and Kain because he was converted through necromancy, plus he's the scion of balance which grants him certain features like the rest of the guardians. Kain is, mmm different, he's not like Vorador but is not like the lesser vampires either.
The first converted vampires were like Vorador but with the vampire purges of the human circle and the suicide of most ancients, the vampire's bloodline decayed until they lost their human form entirely becoming the monsters from Soul Reaver