Decades after the two countries fought a war that killed tens of thousands of Americans and hundreds of thousands of Vietnamese, the two countries have become close allies, united in their distrust of China and especially its actions in the South China Sea. But there are areas of contention, too, especially over Trump's favorite topic, trade.
Trump will meet with the Vietnamese leadership on Saturday in the country's capital, Hanoi, but on his first day there, he'll take part in the Asia-Pacific Economic Cooperation summit in the city of Da Nang, attend a gala dinner and cultural performance, and meet with Vietnam War veterans.
All eyes will especially be on his appearance at the APEC summit, where he delivers a major speech and may meet with Russian President Vladimir Putin.
Potential Putin meeting?
It is that possible meeting with the Russian leader that is drawing the most attention, given the ongoing investigations into any potential ties between Trump's presidential campaign and Russia's interference in the 2016 election. When the two leaders met for the first time in July, there were questions about how much Trump pressed Putin on that interference.
Secretary of State Rex Tillerson has said a meeting is still under consideration but will only happen if there is "sufficient substance." However, Russian leaders have said repeatedly that a meeting will take place, with the Kremlin's spokesman saying Thursday that all that remained was agreeing to the time and format.
Trump will raise the issue of Russia's interference again, according to Tillerson, who told reporters, "It stays on that list" of topics for the U.S.
Trump's other meeting on Friday is with veterans of the war that he infamously didn't serve in, with accusations that he dodged the draft; Trump maintains that he had bone spurs and received a doctor's note that cleared him.
Strategizing over the South China Sea
Among the top issues Trump will discuss with Vietnamese Prime Minister Nguyen Xuan Phuc is China's increasingly assertive role in the region. As China's neighbor to the south, Vietnam opposes China's island-building in the South China Sea and its military buildup on those new islands.
Distrust of China runs deep in Vietnam, stemming back through centuries of fighting, with deadly conflicts as recent as 1979 and 1988.
But the country also worries about an "America First" foreign policy that could mean a less influential role in the region -- or that Trump's flattery of Xi while in Beijing means that he has softened the U.S.'s position with regard to China.
The two countries "affirmed that the South China Sea is a waterway of strategic significance to the international community ... [and] underscored the importance of freedom of navigation and overflight" when Phuc visited the White House in May. But he will be seeking reassurances from Trump that the U.S. will not abandon its ally, as well as possible new steps by the U.S. to counter Chinese influence.
Growing trade ties
Trump's favorite topic will also be an important one in Vietnam, as it has been in his first three countries of his visit -- Japan, South Korea, and China.
Like in those countries, Trump will seek to "remove unfair trade barriers," according to a senior administration official, and shrink the U.S.-Vietnamese trade deficit.
But the Vietnamese have their own bones to pick with America, angry over Trump's withdrawal from the Trans-Pacific Partnership, a multi-sided trade deal that was meant to push back on China and its often one-sided trade practices. Sixty-one percent of Vietnamese opposed Trump's decision to withdraw, among his first as president, according to a Pew Research Center poll conducted in the spring.
In May, Trump and Phuc signed new trade deals that have seemed to soften the blow of TPP withdrawal and drawn the allies closer. The deals, worth $8 billion, included $3 billion of U.S.-produced content that would support more than 23,000 American jobs, according to White House estimates at the time.
Vietnam is the fastest-growing market for U.S. exports, Commerce Secretary Wilbur Ross said in May, rising 77 percent since 2014 to $4.4 billion.
"We think Vietnam is an excellent partner in the region and that there's a lot of opportunity for us to engage in finding ways to work together bilaterally and regionally to promote growth throughout the region," a senior administration official said before Trump's trip.