Vapires are dead creatures and therefore could not produce live young like living creatures do, also, vampires see the world different than the livings, love is not as important as power, so Valen may say she's in love but be only doing it for the power a bhaalspawn could offer. In this case, Valen could slowly become more, err, how to say it, entranced by charnames characteristics than just the power
Vampires are souless creatures, and unlike Sarevok, feel humans, elves and any other 'living' creatures are toys, to be played with, I would think, if charname was a vampier she could fall in love, in an evil type of way, but this would have some sort of affect on the whole child of Bhaal, wouldn't it?