The tomb of King Le Tuc Tong in Thanh Hoa Province has been disturbed, Nguyen Van Hai, the Thanh Hoa Center for Historical Research and Heritage Conservation said.
The suspects, identified as Deng Zhui, 41, and Shen Jiang Yang, 43, from China's Guangxi, were apprehended on May 4 while trying to catch a cab and head for the border, nearly 400 kilometers from the tomb.
Authorities believe the men intended to flee back to China.
The arrests were made after officials at the Lam Kinh historical complex noticed suspicious activity during a routine inspection at 9:30 p.m. on May 3.
They saw irregularities in the area surrounding the tomb, raising immediate concerns.
A subsequent search uncovered metal rods buried five meters underground, believed to be equipment used to detect buried artifacts. These were found around 10 meters from the tomb's perimeter wall.
They also found several water bottles, a mobile phone displaying Chinese characters and the ID card of a Chinese national.
Authorities said that, earlier this year, two men had visited the historical site pretending to be tourists, renting motorbikes and surveying all the tombs in the area, including that of Le Tuc Tong, who reigned briefly in the 15th century before dying at 17.
Authorities suspect that they were the two Chinese who had returned under the cover of night to try and rob the grave.