Nguyen Huu Thanh reduced his stake in the company by 5 percentage points to 0.79 percent on June 6, according to a company filing.
He had bought 508,900 shares on June 1.
In the first five sessions in June, the price rose by 15.4 percent to VND22,850 ($0.99).
Yeah1 came under scrutiny when its chairman and founder Nguyen Anh Nhuong Tong sold his entire 12.89 percent stake on June 1.
Several other major shareholders have also been pulling out since February, including DFJ VinaCapital Venture Investment.
Yeah1 has postponed its annual general meeting twice this year saying more time was needed to prepare important documents.
It reported post-tax profits of nearly VND28 billion last year after two years of losses.
Its contract with YouTube was terminated in March 2020 due to a violation of policies and what began as an operational error has "turned into a real crisis for the company," Nguyen Anh Nhuong Tong once said.