To Lam sent a message of congratulations to the Iranian president-elect Masoud Pezeshkian on his victory in the presidential elections.
The Vietnamese president expressed hope that the people of Iran would continue to gain significant achievements in the construction and development of the country during Pezeshian's administration.
He also welcomed the expansion of the friendly relations and cooperation between the two countries in recent years.
"I would like to emphasize once again the willingness of the government and people of Vietnam to further deepen friendly and multilateral relations and cooperation with the government and people of Iran," he added.
Pezeshkian received more than 16 million votes against former nuclear negotiator Saeed Jalili with more than 13 million out of over 30 million votes cast, electoral authorities said.
"By gaining a majority of the votes cast on Friday, Pezeshkian has become Iran's next president," the interior ministry said.
The runoff on Friday followed a June 28 ballot in a snap election to find a successor to President Ebrahim Raisi who lost his life with his foreign minister Hossein Amir-Abdollahian and others in a helicopter crash in northwest Iran.
Pezeshkian originally ran against a field of five candidates last week, winning the largest number of votes but falling short of a majority which sent him and Jalili to a second round.
MNA