Ah... So it is not as persistent decline as I initially assumed.

And first-stage vampirism is essentially what any newly turned vampire of my kind can be arbitrarily said to have for the first couple to handful of months. During that time, the person will slowly develop the urge to devour other people's blood while just as slowly losing the appetite for and ability to digest most other foods, the societal instincts and feelings would diminish and die out till only the knowledge of having cared about other people at some point remains, and then memories would decay, too, and take even that... Senses become keener, some light-sensitivity develops (my vampires didn't quite burst into flames), teeth elongate, first melanin and then other pigments get eliminated (various albinoids, albinos and leucistic people were regrettably murdered for looking suspiciously vampiric even more often than part-plood palanteri), muscles change, they become stronger and faster, more resilient to blood-loss and damage... Yeah. 
Generally what is considered to be the end of the first stage vampirism is the loss of the last scrap of knowledge of ever having been human, or the loss of any capacity for the related emotions. They will have become feral monsters with little sense or restraint. During the second stage, the physical changes will continue, but the vampire would also start to regain its intelligence and develop the ability to plan and restrain oneself. By the third stage - often considered the final one, though some give very old and far-altered vampires their own category - you have a being who can act just as civil as any human if it wants, but is no more than a thorough sociopath who can only sustain itself on fresh human blood. If some opt to stick with humans, do not be fooled: they only do so because having food follow you around can be convenient. They won't have any scruples about eating anyone, no matter nice they bothered to pretend to be until then. They, curiously enough, also seem to borderline see through or otherwise completely ignore other vampires; same goes for second and late first stage vampires.
 - Basically, the first stage of vampirism is that of losing your humanity and slowly becoming a monster; the second stage of vampirism is that of being a monster; the third stage of vampirism is that of being an intelligent calculative monster with impeccable self-control. And if you want, you can add a fourth stage for slowly having become quite powerful even compared to normal vampires.

(I assume Reniam vampires "black out and wake up a full vampire" - much like Morgan was described to?)