[2]: 241 Through the patronage of Nguyễn Cao Kỳ he was promoted to Lieutenant general and became assistant operations officer and armor commander in I Corps.

It was at this point that Toàn's political connections again became paramount when he was moved south on 10 May to take command of II Corps after the physical and emotional collapse of General Ngo Dzu.

When the conflict settled down to a struggle for the city of Kontum, Toàn cleverly attended to administrative matters and left operational control in the hands of Vann and Lý Tòng Bá, commander of the 23rd Division.

[2]: 148 Toàn was assigned as chief of armor and on 5 February 1975 was also given command of III Corps replacing General Dư Quốc Đống, who had resigned in January after losing the Battle of Phước Long.

[7]: 120–1 After the defeat of ARVN forces at the Battle of Xuan Loc (9-21 April), III Corps began consolidating its forces and prepared a plan which was submitted to the Joint General Staff (JGS) for the defense of the remaining III Corps' territory and the Capital Military District.

[7]: 133 On the night of 28 April Toàn moved his III Corps headquarters from Bien Hoa Air Base to Gò Vấp district closer to Saigon.

Toàn then flew in his helicopter out to the US fleet claiming that he was ordered to liaise with US Marine officers there, but knowing that the Dương Văn Minh government meant the end of South Vietnam he deserted the ARVN.