Known for his pro-Chinese stance, Hoan reached the peak of his career in the early 1960s when North Vietnam temporarily allied with China in the Sino-Soviet dispute.

In 1963, when Foreign Minister Ung Văn Khiêm was replaced by the more pro-Chinese Xuân Thủy, Hoan headed the International Liaison Department of the Central Committee of the CPV.

In the new atmosphere, the leadership found it advisable to replace both Xuân Thủy and Hoan with cadres who had been less conspicuously associated with Lê Duẩn's previous pro-Chinese policies.

Like Trương Như Tảng, who went into exile in Paris, France, Hoan defected and surfaced in Beijing in July 1979 after shaking off political persecution by Vietnamese authorities.

Hoan stated that Vietnam's abuse of its ethnic Chinese minority was "even worse than Hitler's treatment of the Jews" and that its leaders had become "subservient to a foreign power," referring to the Soviet Union.