China is Vietnam's largest trading partner and a vital source of imports for its manufacturing sector, with bilateral trade jumping 21 per cent for the first three quarters from the same period in 2023 to $US148 billion ($A219 billion).
The signing ceremony followed Li's meeting with Vietnamese Prime Minister Pham Minh Chinh.
Li and Vietnam's Communist Party chief, President To Lam, agreed on Saturday to boost economic ties and farm produce co-operation.
The Vietnamese Prime Minister Pham Minh Chinh asked China to boost agricultural trade with Vietnam, open doors to Vietnamese farm produce and enhance customs clearance at borders, state broadcaster Vietnam Television reported.
Vietnam and China also signed a document on updating progress on cross-border railway links related to site surveys.
"The Vietnamese prime minister also suggested the two countries co-operate to implement signed documents on railway links, modern railway development effectively," state media reported.
The two Asian neighbours have repeatedly showed interest in boosting rail links but have not announced concrete plans or the estimated costs to upgrade connections.
The main route relies on tracks connecting Kunming in the southern Chinese province of Yunnan to Hanoi and the Vietnamese port city of Haiphong.
That railway was built by the French during their colonisation of Vietnam more than a century ago and is still in use in Vietnam.
China has replaced its route with high-speed connections.
On Saturday, the National Payment Corp of Vietnam and China's UnionPay International signed an agreement to deploy cross-border payment between the two countries.
