Short answers, pulled from the story.
The State of Vietnam officially proclaimed its existence on the 2nd of July 1949. This proclamation followed the Halong Bay Agreements signed on the 5th of June 1948 which laid the groundwork for a unified Vietnamese government.
Former emperor Bảo Đại assumed the role of Chief of State shortly after the announcement on the 2nd of July 1949. He later created a National Army under General Nguyễn Văn Hinh to defend the state against communist forces.
The State of Vietnam ceased to exist officially when the Republic of Vietnam was established on the 26th of October 1955. Ngô Đình Diệm proclaimed this new republic as its first president following a rigged referendum that ousted Bảo Đại earlier that year.
Five major groups chose to work with Bảo Đại's government including the Cao Đài, the Hòa Hảo, the Bình Xuyên, northern Catholics, and the northern Đại Việt Party. These factions collectively became known as the Big Five while resisting full incorporation into central authority.
Ngô Đình Diệm formally withdrew the State of Vietnam from the French Union on the 20th of July 1954 with Bảo Đại's full support. This action followed the Matignon Treaty signed on the 4th of June 1954 which committed him to establishing a truly independent state.