I hope you have a save game from before you said that, becasue I think you did screw it, yes. Of course she's 'evil' but even evil people want to hear something nice once in a while! The golden rule in any of the romances is to always choose the nicest dialogue option. Just make sure that you don't give Viconia "weakling" answers, but otherwise be nice and interested. If you haven't got a save left you could have a romance with Jaheira in stead: she's neutral so it's not impossible alignment-wise
------------------
Melusine
Your voice is ambrosia