Actually no one knows for sure what the origin of the Vietnamese language comes from. According to this site the most persuasive theory is that it came from the Khmer.
http://vn-style.com/overview/noidung/language.html"There have been many theories regarding the origin of the Vietnamese language. The most persuasive one argues that the Vietnamese language previously belonged to the Mon-Khmer group of the Southeast Asian linguistic system, it was later transformed into Viet-Muong language (or old Vietnamese language) and then separated to form the modern Vietnamese language.In the present-day Vietnamese language, many words have been proved to contain Mon-Khmer roots and to be phonetically and semantically relevant to the Muong language.
Throughout a millennium of Chinese domination and under the Vietnamese feudal dynasties, the official language was the Han (Chinese), but the Vietnamese always demonstrated its strength for self-preservation and development. The Chinese language was pronounced in the Vietnamese way, called the Sino-Vietnamese way of pronunciation, and Vietnamized in various ways to create many commonly used Vietnamese words. The diverse development of the Vietnamese language brought about the birth of a system of writing scripts transcribing the Vietnamese language on the basis of the Chinese characters in the 13th century, called the Nom character.
Under the French domination, Chinese characters were gradually no longer in common use, replaced by French that was considered to be official language in administrative, educational and diplomatic affairs. Thanks to the National language that boasts the advantages of simple figure, composition, spelling and pronunciationm, the modern Vietnamese prose was actually formed and then accepted positive influence from the Western cultural language. The National language characters were produced by some Western evangelists including Alexandre de Rhodes. They cooperated with some Vietnamese to transcribe the Vietnamese language on the basis of the Latin alphabet for using in evangelism in the 17th century. The National language characters were completed and popularized to become a significant cultural tool. In late 19th century, publications were published in the National language characters.
After the August 1945 Revolution, the Vietnamese language and the National language characters have seized a dominating position and strongly developed and established itself as a multi-functional language that has been used in every field, every educational level and has reflected every reality of life. Today, thanks to the Revolution, some ethnic minority groups have their own writing scripts.
The Vietnamese language is characterized by mono-phonology with a concrete, abundant, acoustic and imaginary vocabulary and a proportionate, rhythmical, lively, flexible, symbolic and emotional way of expression, which tremendously facilitates artistic and literary creation. The Vietnamese dictionary published by the Center of Lexicography in 1997 consists of 38,410 entries."