In a meeting with Vietnamese President Tran Dai Quang, who is making a five-day state visit to China, Xi said the two countries should push forward maritime cooperation, accumulate consensus gradually and broaden their common interests.
Quang will attend the Belt and Road Forum for International Cooperation on Sunday and Monday.
Witnessed by the two presidents, China and Vietnam signed cooperative documents in areas like diplomacy, technology, e-commerce, infrastructure and education.
Xi said China and Vietnam should take advantage of the Belt and Road Forum to connect their development strategies.
Noting it is Quang's first visit to China since becoming Vietnamese president, Xi said the visit could energize China-Vietnam relations.
Xi called on the two nations to strengthen coordination to push forward cooperation in all areas and bring practical benefits for the people of both countries.
Quang said the two countries should handle disagreements properly and continue to push forward with maritime cooperation.
Vietnam is willing to expand cooperation with China through people-to-people exchanges and in areas like economy and trade, agriculture, environment, infrastructure, tourism and security, he said.
Liu Zhenmin, vice-foreign minister, said that during their talks, the two presidents discussed the South China Sea "in a positive atmosphere".
According to Liu, the two sides agreed to implement the consensus reached by the high-level leaders of the two countries, make continuous efforts to stabilize the South China Sea situation, and push forward negotiation.
"The talks over the South China Sea issue are completely positive, without any discord," he told reporters.
The two countries will enhance maritime cooperation in "less sensitive areas", such as joint scientific research and exploitation of the Beibu Gulf, he added.
While China remains the largest trading partner of Vietnam, Vietnam surpassed Malaysia last year to become the largest trading partner of China in the association of Southeast Asian Nations.
According to the Ministry of Commerce, two-way trade between China and Vietnam reached $87.8 billion in the first 11 months of 2016, up by 1.6 percent year-on-year.
In the same period, Vietnam's export to China amounted to $32.96 billion, up by 20.8 percent year-on-year, and its trade deficit with China fell by 31 percent.
