If you can't handle reading walls of text then leave now. >.>
I am sure others have suggested this but I would like to suggest that the chance to receive fire if you use the charge to be removed. Looking at how overpowered the Brandish series is now and how limited Fang of Vog really is I believe this would be best for the sword.
Things to consider:
1. Brandish/Levi swings faster then Fang of Vog.
2. Brandishes at lv 10 compared to a lv 10 Fang of Vog do more normal swing damage. With MAX damage I believe the Brandish is 20-30 more damage per hit then Fang of Vog. (Not comparing the 3rd strike of each sword)
3. The Fang of Vog charge attack has a nasty habit of throwing enemies around that aren't shivered or shocked. Except big enemies are subjected to the Fang of Vog's full fury. This throwing enemies around limits the damage Fang of Vog truely does while Brandish type weapon charges can destroy groups of enemies with ease.
4. Fang of Vog cost 40 Almirian Seals while Levi/Brandish weapons can just be crafted. Its a little disappointing having a weapon you have to farm Vanaduke for and learn it has such a nasty drawback. (Well when you are new Vanaduke is hard >.>)
I'm sure there are more reasons, but this is all I could think of in time I was writing this post.
1. It can Set Enemies on Fire on Every Hit, Whereas you need to Charge your Combuster.
2. Its Charge Attack can do MASSIVE Damage if you're next to the Enemy/Enemies the Whole 3 times Strike.
3. Leviathan Blade doesn't have any Special Damage(Shadow, Elemental, Piercing) so it usually Over-Shadowed by other Weapons.
4. The Fang of Vog can easily Clear a Mob of Enemies if you're in the Middle of them.