Wrong.
It was Diem who deposed Bau Dai, the former emperor and legal ruler of the South, and prevented the election between the south and north.
{...
The
1955 State of Vietnam referendum determined the future
form of government of the
State of Vietnam, the nation that was to become the Republic of Vietnam (widely known as
South Vietnam). It was contested by Prime Minister
Ngô Đình Diệm, who proposed a
republic, and former emperor
Bảo Đại, who had abdicated in 1945 and at the time of the referendum held the title of head of state.
...
Emboldened by his success, Diệm began to plot Bảo Đại's downfall. He scheduled a referendum for 23 October 1955 and pushed Bảo Đại out of the political scene, hindering the former emperor's attempts to derail the poll. In the period leading up to the vote, campaigning for Bảo Đại was banned, while Diệm's election campaign focused on personal attacks against Bảo Đại. These included pornographic cartoons of the head of state and unverified rumours claiming he was illegitimate and linking him to various mistresses. The government-controlled media launched polemical attacks on Bảo Đại, and police went door-to-door, warning people of the consequences of failing to vote. After his brother
Ngô Đình Nhu had rigged the referendum, Diệm proclaimed himself president of the newly created Republic of Vietnam.
...}
1955 State of Vietnam referendum - Wikipedia
The US had no legal authority to support Diem and help Diem to prevent fair elections.
To do what the US did in a sovereign nation, is totally illegal.