First, I suppose the party isn't easily going to let the PC die of the disease, right? So there are good chances that they'll have the NPC cleric to remove the disease before it's too late, in which case you have one thing less.
Then they also don't know anything about the curse and about the water-disease. Was it based on a roll or automatic (you could just fudge it)?
I'd say that what the players don't know, doesn't exist until you let them know. So what makes you compelled to have the curse make him a vampire, if you haven't told them yet?
Anyway, my take would be as follows.
- If they don't manage to save him from teh mummy disease, I'd say that you can have him just turns to dust. He's dead, saved from the curse FWIW, and the next PC who picks up the necklace carries the curse on.
- If he's saved from the mummy disease, and he dies from the water-disease, turns into a vampire. After all, the water-disease doesn't state in WHICH undead he turns (Vampire seems out of question, too specific for this), but the necklace curse decides that he becomes a Vampire, or a weird vampiric wight, while everyone else simply becomes a wight.
I don't know about that water-disease, but from your words it seems that the disease itself was only a base for generic undeath, but the necklace specifies which.
BTW, undead are you favourite monster type, right?
