There are no retirement visas to Vietnam. It's tough to stay longer than 3 months. Not many foreigners live here permanently. Most long term foreigners are like me, digital nomads who stay 3 months then move on elsewhere.
Basic western food like peanut butter, pasta is available at supermarkets. The selection is certainly better than on Koh Chang, but not nearly as good as bangkok, pattaya, etc.
Cost of living is similar to Thailand, probably a little cheaper. It depends on the quality of place you are looking for.