Where to Find and Catch Vivillon
From Pokemon Go Wiki
Vivillon are found and captured in Nest and Habitat locations.
Nests
Vivillon Nests are locations that consistently spawn Vivillon.
Habitats
Vivillon Habitats are spawn locations where Vivillon is likely to be encountered, but not guaranteed the way a Nest is.
Additional Tips and Information
Best Nests
If you know a really good nest or any of the nests above is really good, list them here: