President Trump said on Wednesday that the United States had reached a trade deal with Vietnam, one that would roll back some of the punishing tariffs he had issued on Vietnamese products in return for that nation agreeing to open its market to American goods.

The preliminary deal will also indirectly affect China, an important trading partner of Vietnam.

“It will be a Great Deal of Cooperation between our two Countries,” Mr. Trump wrote in a post on Truth Social announcing the deal.

According to Mr. Trump, the deal imposes a 20 percent tariff on all imports from Vietnam and a 40 percent tariff on any “transshipping.”

That provision is aimed at addressing Trump administration criticisms that countries like Vietnam have become a channel for Chinese manufacturers to bypass U.S. tariffs and funnel goods into the United States.

Which products would fall under the higher tariff rate is unclear. It could refer to goods imported to the United States from Vietnam that actually originated in China. But it could also apply to Vietnamese products that use a certain amount of Chinese parts. The deal could include a lower tariff on goods that are made in Vietnam with fewer Chinese parts and materials, and a higher tariff rate for Vietnamese goods that contain many Chinese components.

Vietnam was soon scheduled to face a 46 percent tariff rate as part of the sweeping global tariffs that the Trump administration unveiled on April 2 before pausing those levies for 90 days.