Where to Find and Catch Vanillish
From Pokemon Go Wiki
Vanillish are found and captured in Nest and Habitat locations.
Nests
Vanillish Nests are locations that consistently spawn Vanillish.
Habitats
Vanillish Habitats are spawn locations where Vanillish 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: