Hai, 57, was removed last Tuesday for his role in the illegal acquisition of a TV firm in 2016 by the Ministry of Information and Communications, which runs the corporation.
Nguyen also remains deputy general director in charge of technology area.
Hai has also been sacked from the company’s board after being found responsible for "serious violations" in the acquisition of private pay TV firm Audio Visual Global JSC (AVG).
According to the Central Inspection Commission of the Communist Party of Vietnam, Hai had been personally involved in the acquisition and signed many documents in violation of laws to come up with a deal that caused a significant loss to the government.
MobiFone had made headlines in 2016 when it announced it was breaking into the pay TV market with the acquisition of a 95 percent stake in AVG.
But the Government Inspectorate concluded the deal, which had not been approved by the government, had violated investment laws and caused an estimated VND7 trillion ($307 million) loss to the government.
In a report on the deal last March inspectors said MobiFone had committed multiple violations in proposing the deal and AVG's valuation.
The ministry and MobiFone were responsible for the serious violations of the laws in assessing, approving and going ahead with the deal, the inspectorate said.
MobiFone's after tax profit dropped 26 percent year-on-year to VND1.95 trillion ($86.6 million) in the first half of this year as revenues fell 8 percent to VND14.7 trillion ($653.3 million).
But the country's third largest telco has said with the new CEO taking over it expects to achieve the full year’s financial targets.