A sentient vampire obsessively lusts after one of the PCs. That PC possesses some kind of token or artifact that is taboo to the vampire, however - it's imbued with sentimental or magical power such that the vampire cannot get within 20 feet or harm the owner. This inability to interact with the cause of its desire has done even more psychological damage to an already sociopathic inhuman creature - its obsessed with the PC.
The obsession is bipolar. Half the time the vampire wishes to woo the PC to the vampire's cause and turn them into a vampire to love and cherish forever. The other half of the time, the vampire wishes to kill, torture and rape the PC (in any order).
As an inhuman stalker - all manner of nasty possibilities exist, from friends and family turning up dead or vampires, to bloody messages and obscure anniversaries being celebrated with macabre presents and rituals (e.g. the monthly anniversary of when the vampire first saw the PC naked could be celebrated with a naked body part as a gift).