1. didn't know what to do with Jubilee if she wasn't the equivalent of Robin to Wolverine so they depowered her
2. then they still didn't know what to do, so what the heck let's make her a vampire!
3. They still don't know what to do with her so using the deus ex machina power of the phoenix she is now a mutant again, her powers intact and allegedly no longer a vampire.
At least until she relinquishes the phoenix shard she received....
She doesn't have the shard. All pieces of the phoenix seem to have began coalescing for the current storyline with Jean (if i'm remembering correctly). The phoenix gifted Quire with the shard after his adventure with Thor (where he, Thor, Warbird Gladiator, and Kid Gladiator fought the Shiar gods) and the shard was pretty much his to command. So, Quire shot his one-time, get-out-of-jail-free phoenix shard at Jubilee, it passed through her, activating her old powers, getting rid of the vampirism, and presumably went to Jean for the current story line.