Use Armor of Faith a lot, Protection (Circle) from Evil, Righteous Magic, Draw upon Holy Might, Defensive Harmony etc. There is also a Belt which gives 19 Constitution if I am not mistaken, quite good for her.
Get Mauler +2 and a nice Armor. Basic would be Full Plate with Ring of Protection +2 which is the most powerful combination. Get Shield of the Stars (correct name?). Boots of Speed or in case you need more Missle Protection Boots of Avoidance.
1-2 Sanctuarys and Blade Barriers are nice to have and some Summons like Devas and Skeleton Warriors.
Clerics advance still nicely after 20. Just check their Spellprogression and especially spells per day progression they can cast an insane amount of spells in the 30 regions not counting bonus wisdom which is something a lot underestimate running out of spells much later than most other clerics except maybe a dual class cleric but then again either you dual class early and the gain isn't that much or you dual class late and run around weak quite some time.
Viconia works fine as is in the vanilla game. If you're using Tactics or one of the other difficulty mods you might want to use something like the Level 1 NPC mod to change her to a multiclass.
The game already allows you to create your own party from scratch but then you miss out on all the NPC banter and side quests. I play using the tactics mod and I like to change Viconia to a fighter/cleric. Mage/Cleric has more endgame power but also requires more micro management and takes most of the game to really grow into it's godly power.
I'd prefer she be a dual class like Anomen but that would be difficult to get to work since she's not human. I also change either Imoen or Nalia to a bard and make the other one a MC thief/mage so they actually get some use out of the thief skills.
Just get her a clock of balduran (tactics mode) and the amulett you get at the trader of Usth'Natha and she will her some nice MR of 100! Now she wont get owned by any magic anymore. Second in heavy fights, if you feel her dieing too much, cast this pussy level 1 spell sanctuary and noone will harm her anymore. (Well the are spells that still go thru MR like imprison and sunburtst but she will resist 99% of the spells)
Get her some blunt weapons duall wielded and some melee buffs and she will own like a fighter!
Don't forget about the Human Flesh Armor (if your not going to redeam her) its practically and makes fully resistant to magical damage plus great saves. I like to give her Human Flesh Armor, a good shield, Crom, belt of fortitude and then just cast the right fighting and protective buffs and she will stand in the front lines with anyone.
__________________ Once again time to give someone the boot to make room for Coran. LOL
I'd also add that I enjoy her romance the most of the three biogals. Its actually pretty well written and makes alot of sense especially once she spills the beans about her brother. And if you continue it through ToB there are actual results (depending on how you do things) of her relationship with the PC. I actually remember being pretty madd the first time I finished ToB with Vici as my girl and read her epilog.
Oh did I mention she's hot and has really high natural MR although still not enough to resist me!
__________________ Once again time to give someone the boot to make room for Coran. LOL
She is even more interessting as a drow compared to Drizzt because she really goes through a transformation in the romance especially in ToB unlike Drizzt who is just an Elminster in black skin and in Drow society trying to escape. His romances with Cattie-Brie are also kinda teenie level and not really adult. *yawn
Shame that Viconia's endings after the ToB final battle are pretty sad.
Once again time to give someone the boot to make room for Coran. LOL 
Linear Mode
