Well, we've decided to put this aside for now & I'm just going to play a RCI who works/worked for the White Court as a paralegal.  Thanks for all your input folks.  We may play around at seeing if the original thought can be possible at a later time.

This is an idea I'd want to examine carefully.  First, why does the player want this origin?  What ideas do they have concerning the possible reasons for this to happen?

Looking at the makeup of out little group I had the urge to play a Vampire but not a full blown of either Red or White Court.  I thought it would be an interesting (and yes, I know, TRAGIC) twist in the plot of the game.
 

Consider the repercussions.  If not a breach of the Accords, this incident would be a SERIOUS point of contention between the White and Red Courts in the area.  If a war hasn't broken out, then there would be skirmishes all the time, with hapless supernaturals (including the PCs) caught in the crossfire.

When is there not skirmishes between the Courts?  :-)

Why was it done?  The Red Court doesn't do things 'just because they can'.  Was it a test run to see if they could convert WCV, weakening the White Court?  If so, why did they show their hand now, and why did they convert this character?

My thought is that it is a personal vendetta (ie, not sanctioned by the Red Court) between my character and that particular Red Court vampire.  Basically, the Red Court vampire was "wronged" in some way by my character personally.  I would leave it up to the DM to figure out what exactly brought it on between these two.  (Yes, he's actually excited about the idea. There was an evil grin & some rubbing together of his hands when I mentioned it.)

My personal feelings is that the character is a Red Court Infected; they lose all the special abilities and template of the WCV, as the Red Court demon overwrites the prior curse.

I'd first find out what the players' reasons and ideas were for this character.

My reason/idea is that a member(s) of my characters immediate family would be being protecting her tragic secret 1) from the Red Court so they don't find out that they can infect a WCV (which could, in the long run, wipe out the White Court) & 2) from the rest of the White Court so they don't just kill out right for being "tainted" or end up using her as a bargaining chip for a favor from the Red Court.

Why would the Red Court demon would overwrite the inherited White Court one?  I'm not expecting this character to be very sane since there would be two demons fighting for control.  I don't expect her to live long either but we can give a shot & see how it goes.