Monthly cost of apartment

Of the places you mentioned Hoi An is tight on accommodation - it has places for larger families, for example, house rentals.

Da Nang has good prices (compared to TP HCM) I have seen places renting for $200++++ / month for 2-bedroom house.

Hue prices are a little higher as there are several universities there that attract Foreign lecturers. Things were so tight a few years ago that one uni even built a apartment building in which to house all their guests.

Nha Trang prices tend to be mid-range because it is a resort town that has attracted a gaggle of Foreigners interested in things involving sand and water.

TP Ha Noi, for a capital city, has rental prices that are laughable as many decent places are going for very low prices - around half of TP HCM.

Weather is a consideration. Hoi An, Da Nang, Hue and Dong Ba all have a monsoon season where the winds can do a lot of damage (envision sheets of corrugated iron from roofs flying along at high speed) and flooding (up to your neck, literally)),
TP Ha Noi and northern VietNam is a temperate climate that has cold winters (0C) and hot summers.

The ++++ behind rental rates are for water, electricity, garbage and possibly management fees. You would likely have to deposit two months rent equivalent as a security deposit which means three months rent including First Month when you move in.

Look in CraigsList VietNam, web sites such as this and localised ones for Ha Noi.

There is a shortage of housing throughout VietNam and some governments are even buying privately developed high rises in which to house citizens.

The police control who lives where. Your best opportunities are in the city. I live in a remote city and a luxurious apartment goes for #100 but Foreigners aren't allowed to rent it.

Living out of downtown areas means you need public transportation - which isn't the greatest.

Live from NT: Rental (asking) prices range from around 100 USD a month up to the prices in Saigon (D1).
As a matter of fact, as a foreigner, you will not be able to enjoy anything much below USD 250 a month. Either, it's a shed, not secure or simply miles away from any for of live or in worse case scenario - it's just all together.
Generally, there is no cheap living and less cheap for foreigners in the whole of Vietnam.

What you save on rent, you pay (multiple times) in water/storm damages or extra petrol or loss of property and /or reduced income.
There is a simple structure:

prices are low = higher hidden costs, less easy access to western amenities, very limited employment opportunities
prices are higher = you have commonly less hidden costs, quite easy access to western amenities, limit employment opportunities

hidden costs: tea moneys, medical care, weather issues (wind/water/power), ways to xx and from  xxx....