The crews from the Vietnamese and Chinese coastguards shook hands and took photos when they met last month, sharing platters of fruit and raising their glasses for a toast. Now, they are in a stand-off in the South China Sea.
"The two sides were very happy and united," Lieutenant Colonel Phan Duy Cuong, the operations assistant of Vietnam's coastguard command, said of the April 15 ceremony. "We toasted each other with wine. They went on our boat and we went on theirs."
Ships 8003 and 2007 sailed alongside two Chinese coastguard vessels for three days in the Gulf of Tonkin. A month later, at least one of those Chinese boats has been spotted helping guard an oil rig that Vietnam is demanding be removed from contested waters about 225km off its coast, Cuong said.
The dispute over the rig near the Paracel Islands reflects a renewed chill between the two nations after efforts to draw them closer together, including a flurry of official visits last year. Both China and the US have targeted Vietnam as a potential partner to bolster their influence in the region.
Cuong has been assigned to boat 8003 since it left Hai Phong port on May 5 to patrol the waters west of the Paracel Islands.
Over three days last week, the ship was chased by the Chinese coastguard five times as it attempted to break through a perimeter around the rig. The Chinese ships got as close as 400 metres to the Vietnamese craft, blasting their horns and ordering it to retreat. Other ships were rammed, Cuong said. Both sides have said they used water cannons. China has accused Vietnam of ramming its ships.
"We were working together just days before, but now there is a line dividing us," Cuong said on board boat 8003, which carried a crew of 50, plus 100 live chickens in a pen at the stern. "I'm very sad." While on the joint patrol in April, the boats together inspected Chinese and Vietnamese fishing boats.
Anti-Chinese violence flared in Vietnam last week after a US$1 billion deepwater rig owned by China's state-run China National Offshore Oil Corporation was parked off the Vietnamese coast. Hanoi says the rig is in its 200-nautical mile exclusive economic zone and on its continental shelf. China says the rig is operating completely within its waters.
Vietnamese Prime Minister Nguyen Tan Dung said his government was considering various "defence options" against China, including legal action, following the deployment of the oil rig.
The White House said yesterday it would support the use of legal action by Vietnam.
"The United States supports the use of diplomatic and other peaceful means to manage and resolve disagreements, including the use of arbitration or other international legal mechanisms," White House spokesman Patrick Ventrell said.
In late March, the Philippines formally submitted a case to an arbitration tribunal in The Hague, challenging China's claims to the South China Sea. It is the first time Beijing has been subjected to international legal scrutiny over the waters. Beijing has refused to participate in the case and warned Manila that its submission would seriously damage ties.
Deputy Prime Minister Vu Duc Dam said yesterday that Hanoi was closely following the case brought by the Philippines.
The cooling in ties is less about China picking a fight with Vietnam and more about it warning off the US, according to Tan See Seng, an associate professor at the S.Rajaratnam School of International Studies in Singapore. "The reason for the schizophrenic quality of Chinese behaviour, I suspect, has to do with what China thinks US intention and strategy toward it might be," Tan said by e-mail. "China's big worry is the US and its partners will block China's access to strategic trade routes through the South China Sea."
Out on the South China Sea last week, boat 8003's radar screen showed about 60 Chinese ships facing off against half a dozen Vietnamese boats. Cuong's ship got within three nautical miles of the oil rig on May 6 before being turned away and has not been that close since.
"The first day we got there we saw all these ships turn on their lights," said Bui Son, a crew member in charge of artillery. "It looked like a city. We were so surprised to see such a heavy presence of Chinese ships in Vietnam's territorial waters. We were shocked."
As he spoke, in the distance the rig rose from the sea like a giant tower, with a platform on a red base holding several cranes. At night it glows and can be seen as far away as 12 nautical miles.
The crew of boat 8003 has seen two Chinese missile-launching ships in the area, while Chinese aircraft have flown over at low altitude. Vietnam state media reported a Chinese submarine in the area.
Anti-China protests in Vietnam last week morphed into attacks on factories operated by companies from Taiwan and Singapore, leaving at least two Chinese nationals dead and scores of businesses damaged. That prompted Nguyen Tan Dung to instruct provincial governments and security forces to take "quick actions" to stop the violence and prevent protests. At the weekend China sent five ships to Vietnam to evacuate its citizens, more than 3,000 of whom had left by Saturday. More than 3,500 Chinese workers boarded two of the ships in the central province of Ha Tinh on Monday.
Things had appeared on a more positive footing last year, as President Xi Jinping and Premier Li Keqiang made a series of visits to Southeast Asian nations, pledging investment and bolstering trade ties.
Vietnam and China last June set up a hotline between their leaders, and expanded a 2006 agreement to jointly explore for oil in the Gulf of Tonkin. Li visited Vietnam in October, where he and Dung pledged to boost "political trust", signing a memorandum of understanding for a cross-border economic cooperation zone and agreeing to open trade promotion offices.
Vietnam's President Truong Tan Sang visited Beijing last June, where he met Xi and they agreed to push "pragmatic cooperation" on areas such as defence, Xinhua reported.
The attacks on workers in Vietnam prompted Foreign Ministry spokeswoman Hua Chunying to criticise the Vietnamese government for "indulgence and connivance toward domestic anti-China forces and criminals".
The oil rig's presence off Vietnam's coast was "very normal behaviour", General Fang Fenghui , chief of the People's Liberation Army's general staff, said in the United States the same day. Vietnam had sent ships to disrupt the drilling operations, "and that's something we aren't able to accept", he said.
China's growing self-confidence on the international stage, coupled with Xi's "tough" style, means the relationship with Vietnam could stay tense, according to Arthur Ding, a research fellow at the China politics division of the Institute of International Relations in Taipei.
"I don't think you're going to see it escalate into a war," Ding said. "China and Vietnam are still self-restrained."
Out on boat 8003, Son said he was not afraid of the larger Chinese ships.
"What I do worry about is that our friendship is fading and we are losing trust in each other," he said. "That's the bigger loss for the two countries.
"When we said goodbye, we promised we'd see each other again. Now we see each other in this very difficult situation."
Additional reporting by Reuters