If know console commands, you can fix it (just if you have finish Dawnguard).
Open console
help "Serana" 4
you will get some lines and among them one about a quest she can do to cure her vampirism and you will get the ID of this quest (XX005044)
Close console
make a save
open console
Setstage XX005044 0
close console
try to talk to Serana she will have no dialog option and say something about "I have to do it now"
go to dawnguard.
wait there 72 hours +
Serana is back and cured
hire Serana
job done
She has strange yellow eyes and the XX there is the number Dawnguard is in our load order.
There is no "fix" to this because it isn't a bug. She tells you she doesn't want a cure, she convinces herself what she has is a gift that she would never give up, and that is that.