And so the neverending debate continues. Good thing too, I've missed it! I completely agree that vampires shouldn't be humans with fangs and that Valen shouldn't become 'just another
NPC, only much more powerful'. Thats why Im strongly against the option to redeem her. Anyone mention turning her back to human and I'll put a stake through your right big toe(it hurts more than you'd think... or so Im told).
However, I dont think Valens alien mindset makes it impossible to romance her. Consider the stuff having Valen in your party will lead to: killing a bunch of innocents, fighting the forces of good and order, making deals with demons, and generally raising hell. Hah! Good fun! You'll end up with a rep of 1 and lots of blood on your hands(unless Valen licks it off... now THATS a lovely image!) However, there's no way a good or lawful person would keep Valen around. While a good person migth tolerate Korgan or Edwin because their skills were needed. They talk big and have bad attitudes, but mostly do what they're told. Valen runs around killing children, even when you tell her to behave. See the difference? Only someone fairly evil and unconcearned with anyone else would keep Valen around.
So what we have is an evil Bhaalspawn and a sociopatic vampire. She has connections to the negative plane? The PC has connections to the lower planes. Sounds like a match made in the abyss! In fact, I'd go as far as saying that Valen seems like a far more suitable romance option for an evil PC than any of the original romances. Viccy is all talk and bitchyness, but lets see her handeling the slayer in bed!

Ofcourse, as its been previously stated, romancing Valen wouldn't be pretty. Perhaps there'd finaly be a romance where you could tell your vict.. err, partner to shut the f*** up and do as you say, and the romance would actually progress from it! No more damn therapy or whining, just sex and violence! Could make for some interesting lovetalks too. While Valen doesn't come across as the deepest person, she has a decent int(and it keeps improving), so she should be able to handle some advanced philosophical debates on the nature of power and other such things.