Thanks for the input people!
The situation is a little bit odd - there isn't actually any other players at the table, this is a one-on-one game I'm playing with my GM.
What we needed was a refocus on the basic question of "do we have an interesting outcome for success or failure?" We realized that neither of us sees this as something interesting to fail, since there is no logical result of failure that still leaves the character alive and we have too many more stories we want to explore with this guy
So, instead of playing out the ritual itself we'll play out the (hurried) preparations, and as soon as the wizard sits down in his circle to start his ritual we cut to the other people in the building - the ones that have to keep the ravenous, old, powerful, out-of-control RCI away from the ritual
without killing her or allowing her to kill any of them.
For the curious - the end effect of the ritual is a long-term thing, making the house a safehouse for the Fellowship of S:t Giles. The "sunlight" will work as both a help in recuperating after drawing on the vampire part too heavily for too long, as well as protection if the house would be compromised - most Rampires would be completely incapable of even entering the house, but that would come as a surprise since there is no visible light coming out of the house.
In the short term, it will help subdue the above mentioned RCI enough for her to not rip out throats and instead get time to cool off and regain control of herself.