In Western Catalan, unstressed vowels reduce to five: /e ɛ/ > [e]; /o ɔ/ > [o]; /a u i/ remain distinct. This reduction pattern, inherited from Proto-Romance, is also found in Italian and Portuguese. Some Western dialects present further reduction or vowel harmony in some cases.
What do unstressed vowels reduce to in Western Catalan? Which letters remain distinct? Where did this pattern come from? In what other languages is this pattern found? What do other Western dialects sometimes  offer?