Well, as a fan of Vivillon, let me share you guys the set I've been using:
Hyper Eyes Vivillon
Vivillon @ Wide Lens
Ability: Compound Eyes
EVs: 4 Def/ 252 SpA/ 252 Spe
Modest Nature
- Hurricane
- Substitute
- Sleep Powder
- Quiver Dance
Since defensive stat of Vivillon is really bad, most of the time, he can be OHKO. Setting up and blowing out enemies away is his better choice. After the boost of Compound Eyes, Hurricane has 91% accuracy and 97.5% for Sleep Powder, and not 100% should be considered "Risky".
When I use Vivillon to wipe out someone's team, they always wonder why that 91% Hurricane ALWAYS hit, and the trick is Wide Lens. With Wide Lens, Hurricane became an reliable STAB move with enormous power that has 30% confusing affect and sure hit harder than Volcarona.
Substitute works great for Vivillon to help it feel free setting up while blocking status effect, especially Thunder Wave. Sleep Powder is an annoy move, but don't depend on it since the sleeping mechanic have changed and furthermore, most of the time it will be 1 turn Sleep.
Checks and Counters:
Just Steath Rock like Volcarona! Rotom is a big problem for this Vivillon since it take time to finish and if Sleep Clause is in effect, Vivillon can just hurt it once and then kiss the battlefield good bye. Talonflame hit Vivillon hard, too, and it's OHKO for sure. Magnezone block this Vivillon completely, but HP Ground for Vivillon is not really as useful as Substitute, so it's not worthy.
Vivillon is a good late game sweeper, after clearing most of it threat. If you can combine with Sticky Web, making Vivillon outspeed most of Pokémon, Vivillon is really dangerous.