Floods, resulting from the fall monsoons, affect the mass of land between just south of Hoi An right through Da Nang, Hue and as far as Dong Ba.
If you have the right mental attitude you can over come the effects of flooding, then comes the physical discomfort and deprivation and finally the clean-up.
Think positive, it ONLY lasts 3-4 months! That's in the bad years.

Spot your house - Hoi An
Still Hoi An has it easy compared to Da Nang.
If you have studied the lay of the land, you will have noticed it is predominantly flat. This makes for equality - seeing how water seeks a natural balance.
The worst aspect of flooding in VN is the fact that they do not have separate waste sewers for toilet sewage and street/rain water. So when the floods come most all of the sewers back up and over flow in to the streets and houses and buildings. Unfortunately 99.999% of the houses in VietNam don't have anti-back flow flaps so flood water has an ingress into most homes.

Can't find the key hole
Another feature of VN building is that electrical outlets are often at chest height. This is no accident, it means they are usually above the flood line.
Back to Hoi An. There is the 'ancient town' where most of the houses are made from wood. You will also find there are holes drilled in the floors. The holes allow the water in and stop the buildings from turning in to boats and floating away.
The wooden construction is actually very smart - given how much water trees contain naturally. The annual soaking does little damage to the structures. The streets in the Ancient Town do turn in to rivers, sweeping away the debris, motorcycles and chairs. Winds can get high with signs, roof covering (the sheet corrugated iron are the most dangerous) if not secured, flying around.

Check out the water against the road sign
The land between Hoi An and Cua Dai beech is flat and only (just) above the river water line.
During the monsoons, sea tidal surges increase the water line even as far inland as Hoi An which further adds to the difficulty of getting rid of the water.
Along the beaches of Cua Dai, at low tide, you will see there is a two-metre 'step' in the level of the stony beaches, this is caused by the wave action of the monsoons.
Preparation is essential. Extra bottled gas; plenty of batteries and flash lights; candles and matches; waders; plastic buckets you can use as Porta-Toilets when the water is around; good quality rain coats (you can buy the from marine suppliers in Da Nang. A portable generator will give you some semblance of civilisation and keep the refrigerator running (don't forget the gasoline/petrol).
You will need copious amounts of bottled water (for drinking and cooking) and visiting the Metro store in Da Nang will allow you to buy box lots of noodles and other supplies.
Old hands in the flood areas have their motorcycles modified so that the carburettor/carburetters are extremely high, along with air intakes, so don't be riding an unmodified bike in the water1
The Vietnamese have survived these annual upsets for hundreds of years, and undoubtedly you can too. And then you can wear a T-shirt saying I survived the Monsoons.
I have experienced the storms in Hoi An, Da Nang and Hue (where water routinely is over two metres in some parts of town) and Da Nang was easily the worst with the wind driven debris.