Definitely viv-ee-on, most likely a combination of vivid and the French word for butterfly 'papillon', pronounced in a similar fashion. There are a handful of European languages where a double L makes a Y sound. Rattata's name is pronounced with more variation, but according to the good folk at Bulbapedia, the name is most likely a combination of rat, attack and rat-a-tat, which would suggest the first and last syllables are stressed.
God forbid if they start making Pokémon with Welsh names...