The global variable "JAHEIRAROMANCE" determines how long the romance you and a particular character had been going on. I think the global variable that will cause Bodhi to turn your lover into vampires is when "JAHEIRAROMANCEACTIVE" is set to 2, same goes for the other NPCs that have this option (ie. Aerie, Viconia, and Anomen).
As for the meaning of "JAHEIRAROMANCEACTIVE", (or for any other characters, for this matter) this is my interpretation:
1 = still working on it, eventually it will turn into 2 or 3, depending on your dialogue choice
2 = romance script ends (almost ends), technically the two of you are together
3 = it's over between the two of you (eg. when you tell Aerie to whine somewhere else)

FREE LUUUUUVE BABY !!

SPOILER ABOUT ROMANCE AND BOHDI
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I was spoiled by accident one time. I heard that Bohdi would make Jaheira, Aerie or Viconia to a vampire if you have a romance with one of them, but she haven't and I have already killed Bohdi (for good this time). I have talked to Jaheira twice after the harpers attacked me in the woods. I checked shadowkeeper, and it said that the JAHEIRAROMANCE was above 3000000. Does anyone know when there is going to be a real romance?? Jaheira doesn't know if she likes the harpers or if I will betray her. I have always said the right things.

This is one long romance the gamemakers have put together. The variable is set on 1, so there is something still going on. I tried to speed up the romance by resting a lot, but she doesn't talk. I suppose I have to run around the world if she is gonna talk more. I will have to put Bohdi aside for now. I wanna see this vampire story.

Watch out and make sure Jaheira doesn't start repeating herself. I ran into that problem earlier and the board purged that post. Some how cheating screws up the romance thread

Try [ctrl] + [A], I heard this code is suppose to let one game hour pass each time you use it.