When is Vivillon's form calculated?

Question

Is it calculated when you catch the Pokemon, or when your Spewpa evolvess?

Answer 1

Vivillon's pattern is determined by the region you live in, so any Vivillon caught in your game will always have the same pattern, and any Spewpa will evolve into a Vivillon of that pattern.
So Vivillon's pattern isn't determined when you catch it or evolve it, because it is already decided what pattern Vivillon from your game will be.

Answer 2

Vivillions forme depends on the real-world geographic location set on the game the Pokémon was generated in.

Source: http://bulbapedia.bulbagarden.net/wiki/Vivillon_(Pokémon)

Map for forms: http://i.imgur.com/FmJI2li.png