I'd still assert that Sleep Powder is better; Tailwind is not terribly useful in single battles, especially in-game. Sleep Powder is essentially free turns (barring the 2.5% chance of a miss) and it should let you setup to +2 67% of the time. If you're matched against a special attacker, you can setup anywhere from +2 to +4 before the enemy can attack, and with the +SpDef boosts, you can easily setup to +6 and sweep.
Assuming that Vivillon can outspeed the opponent's lead Pokemon, even if it only sets up to +1 or +2, it can outspeed the next Pokemon with a 97.5% accurate Sleep Powder and continue setting up. I don't understand what's the point of Tailwind on this set - it doesn't help Vivillon because it's redundant with QDance, and the boost is not only transient, but it requires switching out after a turn of leaving Vivillon totally vulnerable.