I dont think you can turn Viconia actually good ? You can turn Sarevok chaotic good, but the best you can do with Viconia is AFAIK true neutral. Also I think you have to romance her to archieve the later and I never did that, because she would leave the party once reputation crossed beyond 18.
I never figured out how to play BG2 properly as evil, or even neutral. My reputation always shot up to 20 very quickly. Murdering random innocents could bring it down but that was no fun at all.
BGs lean (and early Bioware games in general) very, very heavily toward good reputation, and if you are questing your reputation will skyrocket to 20 in no time. But no fear, the game gives you Ring of Human Influence after your first likely adventure for a reason. Put it on, and see all companion quibbles dissapear (that or create a high charisma character to start with).
While not exactly fixing a problem, I used to prefer playing BG2 with virtue mod, which split reputation into two: virtue was what your team thought of you, reputation is how the outside world saw you. So quest like freeing slaves would increase your virtue which would appeal to good aligned companions but mayhem you caused would lower your public reputation.
