Do you want a big city? You mention Bangkok, Ho Chi Minh, Singapore.
But if you like Vietnam and can have a smaller city, and dislike motorcycles, try Hoi An. It is beautiful, far fewer motorbikes especially on the outskirts (you can cycle on a bike!), and close to Da Nang if you want a semi-large city (a million people.)
I like Da Nang and Hoi An for a short holiday, but it's not big enough for my tastes. I do like Vietnam, but I like Thailand a little bit more. Thanks.
But if you like Vietnam and can have a smaller city, and dislike motorcycles, try Hoi An. It is beautiful, far fewer motorbikes especially on the outskirts (you can cycle on a bike!), and close to Da Nang if you want a semi-large city (a million people.)