As fantazero said, ultimately, it it up to you. Or rather your group. Ask them what they think would be cool to happen here.
I could see something like this be a point where the changeling is forced to choose. If he chooses faerie, that will ward off the vampire. If he chooses mortal, he will be turned into a vampire. Do or do not, there is no halfway here.
Or he becomes a thirdish...thing. Could be interesting to see what happens, if vampire and faerie and human try to fight over the dominion over his body. The unique situation of being part faerie might actually allow him to shed the vampire bits along with the faerie stuff, to get around the whole "there is no cure" issue. If you want to take it ad absurdum, add something else that is trying to transform him. And more. And more. If the player is cool with this, of course. Doesn't all have to be reflected in powers, of course, but it would be some disturbing voices in his head