I used a Vivillon on my second playthrough of X. If you somehow manage to set up with Quiver Dance it can be pretty powerful, but its frail so it would easily get KO'd by the opponent. For me it was good early on, but just got outclassed by other Bug types. I'd recommend using a different Bug type. (note: I haven't done much Pokémon stuff lately so some of these descriptions may not be the best)
Pokémon you can get without trading:
Scolipede is a good bug type. Its level-up movepool isn't the best (you'll be stuck with Steamroller and Venoshock for a while), but once you get some TMs its a monster. If you want, you could give it Swords Dance to set up and sweep with moves like X-Scissor, Poison Jab and Earthquake. Venipede is also available quite early on, you can get it on Route 6.
Yanmega could also be good, it gets Speed Boost and has some pretty decent stats. It doesn't get any STAB moves until level 49 where it learns U-Turn though. Yanma is obtainable a bit after Venipede on route 10.
Heracross is also an option. It has a good attack stat and some decent speed. It gets Megahorn and Close Combat at a decent level (37 for Megahorn and 43 for Close Combat) which is good. If you can get someone to trade you a Heracronite (its only obtainable post-game if I remember correctly) then you could give it moves like Pin Missle, Bullet Seed and Rock Blast. Its found on Route 12.
Pokémon you need to trade for:
Volcarona is an amazing Pokémon in my opinion. It evolves really late game (Level 59), but its totally worth it. Volcarona has a super high Special attack stat and good Special Defense/Speed stats too. It gets a bunch of good moves but it learns a lot of them at a high level. If you can go to the move relearner, then you could get stuff like Fiery Dance, Hurricane and Quiver Dance without having to wait until a super high level.
Galvantula is pretty good too. It has a good Speed stat and decent Special Attack. Its also an electric type, which means Flying isn't super effective on it. Galvantula gets Compound Eyes which allows it to use Thunder and not miss super often. If you want, you could also use Sucker Punch since its attack isn't the worst.
Scizor is a very powerful bug type. It has a high Attack stat and good Defense/Special Defense. Its also a Steel type which means that its only weak to Fire. It can also get Technician which powers up moves with a base power below 60 which could help out quite a bit. Scyther is available super late game on Route 21, and you need a Metal Coat to evolve it into Scizor.
...But anything can work in-game, so it doesn't really matter what you use.
I hope I helped!
