A fine point by FluffigtDjur, that Valen might, for her own reasons, be tempted to seek a romantic interest with the PC.
It could start out sincerely(if Charisma is high enough) or with an ulterior motive(from Bhaal blood to leading the PC on), and slowly develop. If Valen is so interested in power, then the rising force of the PC should eventually pique her interest, and, against her will, she begins to develop genuine feelings for the PC. The climactic moment would be in Spellhold where Bodhi betrays Valen, and later where Bodhi actually flees the Slayer incarnation, plus Dace Sontane. At that point, her loyalty to Valen should certainly be shattered, and even an evil and unredeemable Valen should start trying to be on the winning side.
Alternatively we could have some fun(insane laughter).
The PC is attracted to Valen(the fictional record of vampires as seductive creatures is clear) and eventually reaches out and shows some interest in her. At this point some random number generator goes into effect.
In terms of percentile, assume that if the dice come up with 000, Valen is genuinely moved and attracted to the PC and becomes his lover and loyalist.
On 001 through 009 matters proceed normally.
On all others(010 through 999) the party guard has been dropped and things get...interesting.
In the final showdown with Bodhi, assume a 50% chance that Valen has been 'snacking around' and any given party member's free will has been weakened to the point of automatically being charmed by Bodhi!
In addition, assume that, to eliminate the competition, Valen AUTOMATICALLY sank her teeth into Jaheira and/or Viconia, if either are with the party. Heck, have her disappear from the party for several hours one night. While away she pays a visit to Aerie as well. Later Bodhi's minions grab Aerie for the showdown.
If our PC can win against these odds, great! He deserves applause and the chance to clean up this mess.
If not, we can close with some movie where Bodhi gets her final revenge by sending the vampires Aerie, Viconia, and Jaheira to feast on him.
Hmm, can this idea get even more perverse? Sure!!!