SOS Encounter: Mount Lanakila (11%, Only in Ultra Sun & Ultra Moon), Tapu Village (10%)
Its base catch rate is 255 out of 255 (higher is easier); the catch rate calculator works out your odds with each ball. For the look, the best colour match is Dive Ball.
Evolution: Vanillite → Vanillish (Level 35) → Vanilluxe (Level 47).
Vanillite in every game · Pokémon Sun, Moon, Ultra Sun & Ultra Moon interactive map (Alola)