It looks like you are nearing the end. Generally I wouldn't advice dropping characters at this point because if you recruit someone anew he/she will be probably lower in level than your party at this point (even with the lvl adjustments mad by the game). But if you still have some quests to build up XP you can of course ditch Viconia because of her alignment and add somebody else.
You seem to have a very balanced party.
Quote:
Minsc&Mazzy = 2 tanks
Jan = Thief
Jan&You = Mage
Anomen&Viconia = Cleric/backup tank
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because all the key players are present twice (thief is not THAT important)
If you really want to lose Viconia and add a good NPC I'd consider Keldorn again (his True Sight rocks) or Valygar (nice equipment and don't underestimate Stalker-backstab) who you have to deal with anyway for the Sphere.
BTW what did you do to Imoen? I never leave my sis alone when I'm playing good.