Eh, hard to say. The books seem to indicate the soul gets destroyed and the demon takes over everything...memories remain, but the person is gone (I assume Susan was still in the process of changing, to an extent, when she was killed by Harry or perhaps Susan was just confused and that's why she didn't fight it).
Like Laciel's shadow, you could potentially give a Red Court Vampire something like Free Will by injecting soul-stuff into it (Harry did this by giving some of his will/soul to the Lash. A magic ritual designed to preserve the soul and merge it with the demon could arguably work (Complete Transformation Ritual of some sort, effects activating upon a kill). You'd still have to deal with hunger and other issues. I'd say this is a 0 Refresh Ability, really, since honestly it seems like flavor to me. Person wouldn't really be human anymore, and would be vulnerable to sunlight, have problems with thresholds, etc. I'd say killing people by feeding would have bad consequences too...you'd have to figure that out..maybe something like Lawbreaker as they become more demonic.
You might want to add demonic co-pilot to represent the dark side of the Red Court nature.
Hmmm, I"m of two minds about this. It's an added mechanism to complicate what is essentially a role-playing challenge, but it could be a good reminder about the tenuous hold a Rampire would have over its ability to cooperate with others.
I *can* see one type of over-the-top situation in which a fully turned Rampire might be cooperating with "good guys": getting involved in a conflict against an even bigger threat to the world, like Outsiders - like how Lara and Thomas are fighting the Oblivion War as part of the Venators, a secret group of the Venatori Umbrorum dedicated to eradicate all knowledge of Outsiders - though it appears in the Red War that the Rampires may be allied with Outsiders, so this would have to be a Red who "saw the light"