While you may be underlevelled, it's very easy to level up with an abundance of Audinos, so that shouldn't be too much of an issue. All your current Pokemon are very viable to take out the Elite 4, but I would still recommend replacing Serperior and Vanilluxe, and possibly Unfezant if you so choose.
Serperior to Volcarona: Yes. Volcarona gets Quiver Dance at level 52, plus other good moves around that frame like Heat Wave and Silver Wind. Fire and Bug can hit a large portion of the Unova League super effectively.
Volcarona @ anything
Ability: Flame Body
- Quiver Dance
- Flamethrower / Heat Wave
- Bug Buzz
- Psychic
Unfezant to Braviary: Probably not, but you can if you want. The Flying type isn't that useful in BW's Elite 4, only hitting Marshall and a bit of Alder's team super effectively, both of which can be taken out by strong neutral STAB moves anyways. Unfezant should be perfectly fine, and Braviary would be a waste of time to grind, but you still can if you want.
Braviary @ Silk Scarf
Ability: Sheer Force
- Brave Bird / Fly
- Return / Strength
- U-turn
- anything
Vanilluxe to Mamoswine: Yes, yes, a thousand times yes. Ice + Ground is one of the best offensive typings in the game. It hits quite a few Pokemon for super effective damage, and even more for neutral. Mamoswine is far superior to Vanilluxe, as it's slightly bulkier, and has better moves.
Mamoswine @ anything
Ability: any
- Earthquake
- Ice Fang / Ice Shard
- Strength
- Return
Hope I helped!
