There were many former Viet Minh soldiers in the Army of the Republic of Vietnam (ARVN), some at its highest levels and many more at mid and lower levels. Just like South Vietnam or the Republic of Vietnam, ARVN was a new name for an older organization called the Army of the State of Vietnam. Both were part of the so-called Bao Dai solution that emerged in 1949-1950, backed by France and non-communist Vietnamese parties.
If there's one thing I'd like to stress, it's that more Vietnamese died fighting for the State of Vietnam, against the forces of Ho Chi Minh and the Democratic Republic of Vietnam. In a fundamental way, the French Indochina War is a misnomer.
The non-communist political parties and armed groups (there are quite a few, from mass religious organizations to national political groups and local organizations) were anti-colonialist and opposed to the return of France. They were part of the Viet Minh 'front' that initially aimed to unite all Vietnamese. But as the members of the Indochinese Communist Party moved to consolidate their control over the Viet Minh, attacking independent units not-subordinate to their control, the fracture between these groups widened quickly in late 1946. Many Vietnamese would, only then, migrate to the non-Viet Minh solution -- the Bao Dai government, officially the State of Vietnam. From this perspective, they were a more moderate nationalist solution, working with France to achieve independence for Vietnam, analogous in a way to India in the British Commonwealth.
From 1951-52, both the DRV and State of Vietnam (both claiming total control of Vietnam) enacted mobilization measures (the draft) to pull young men into their respective armies. Each side would have over a hundred thousand Vietnamese under arms by 1954. Many former Viet Minh were drafted into the State of Vietnam Army, and State of Vietnam Army members would defect at times and join the Viet Minh. Often it has little to do with ideology. People are tired of the terrible conditions in the Viet Minh/DRV army, or their family members were persecuted by the Viet Minh. In any case, there was a good deal of turnover between the two sides. Some of the semi-autonomous auxiliary forces switch sides five times or more!
ARVN would come into being after October 1955, when the South Vietnam president renamed the government (having deposed Bao Dai) and re-christen the State of Vietnam army as ARVN. So, yes, quite a few ARVN had previously served in the Viet Minh. ARVN, as the Viet Minh, were distrustful of France and welcomed their departure in 1956.