Such an evil plan..but such naive thoughts to divert Yuan River and Mekong River and cut off Vietnam's water supplies? You should really do some research before posting such comments. It might make you all sound a bit smarter.
Hanoi inhabitants receives 80% of its water from groundwater. The other 20% is from surface water comes from the Gia Lam plant and the Thang Long North-Van Tri. This is more than enough for consumption and for agriculture use. Duong River alone can satisfy the population demands and replace the groundwater. Other potential water sources includes Ca Lo Rivers, Cau River, and To Lich River. For South Vietnam, 93% of drinking water comes from Saigon River and Dong Nai River. Vietnam's dry season is March-December and wet season is April-November. Water balancing, management and and improvement of water quality is already within Vietnam's reach.
Diverting or building a dam in Yuan River will increase flood risk, because dam failure is quite common. Flood will destroy houses and buildings, and carry soil away from valuable farming land. Floods can also contaminate drinking water and lead to diseases.
Concrete usually harden after 7 days but the chemical reaction that causes it to cure may slow down, but never quite stops curing. Rapid curing can lead to excessive thermal expansion, which will cause cracking and voids. Water erosions are typical and requires frequent costly maintenances. But China is rich now and she can afford this.