Tokyo and Hanoi are expected to forge closer security and economic ties when Vietnam's top leader, Mr Nguyen Phu Trong, makes a four- day official visit to Japan this week.
During talks with Mr Trong, Japanese Prime Minister Shinzo Abe is likely to pledge fresh economic aid.
The leaders are also expected to discuss greater security cooperation, with Hanoi asking for more patrol vessels to counter greater Chinese muscle-flexing in the South China Sea.
Japanese scholars see Mr Trong's visit, starting on Tuesday, as a move by Hanoi to maintain balanced diplomacy with major regional powers following his trip to China and the United States earlier this year.
Besides talks with Mr Abe, the general secretary of the Vietnamese Communist Party is scheduled to have an audience with Emperor Akihito on Wednesday and a meeting with Japanese business leaders on Thursday, according to the Japanese Foreign Ministry.
Citing Vietnam's leadership change at a party congress slated for early next year, a foreign ministry official said: "Following his trips to Washington and Beijing this year, Mr Trong has expressed a desire to visit Japan (before he steps down). The visit carries the message that Vietnam attaches importance to Japan and vice versa even after the leadership change."
At their talks, Mr Trong and Mr Abe are expected to affirm Japan's increased assistance in enhancing Vietnam's maritime law enforcement capabilities at a time when China is pushing its territorial claims in the South China Sea.
Most recently, the Chinese built an artificial island in the hotly contested Spratly Islands.
Japan has provided Vietnam with two of the six used patrol vessels it pledged last year. Hanoi will be asking Tokyo to offer more vessels.
Vietnam, together with Brunei, Malaysia, the Philippines and Taiwan, has overlapping territorial claims with China in the South China Sea.
Japan is itself embroiled in a row with China over a group of East China Sea islets administered by Tokyo, but claimed by Beijing and Taiwan.
China, which claims nearly all of the South China Sea, has rejected American, Japanese and other third-party intervention. It insists on resolving disputes through direct negotiations with claimant states.
Mr Abe is also likely to pledge fresh economic aid to Vietnam as part of a five-year, US$110 billion (S$155 billion) "high-quality and innovative" infrastructure investment initiative for Asia that Japan advocated in May, according to Japanese officials.
The initiative matches the size of the planned launch this year of the China-led Asian Infrastructure Investment Bank with authorised capital of US$100 billion. Vietnam and other countries in the region have signed up as founding members. Japan and the US did not opt in.
On Hanoi's efforts to build closer ties with Japan and the US, Dr Mie Oba, a professor of international relations at the Tokyo University of Science, cited reasons why Japan should not expect too much.
"Japan should not expect Vietnam, like other South-east Asian states, to stop performing a delicate balancing act and move closer to the Japan-US alliance," she said.
She cited Hanoi's close economic ties with Beijing and traditional links between their ruling communist parties. China is Vietnam's biggest trading partner, while Japan is its largest aid donor and about 1,400 Japanese firms operate in Vietnam as of this year, according to Japanese government data.
"Vietnam probably regards Japan as one of the variables in its diplomacy," said Dr Oba. "I would assume that with the upcoming visit, Mr Trong will appeal to the world that Vietnam will strengthen ties each with Japan, the US and China."