The Vetus Latina is the Latin version of the bible, probably the oldest form of the New Testament. It was known as the Latin Vulgate because it was the most commonly used bible. Then in 382 Jerome under the order of the Pope compiled a new version by taking many of the Vetus Latina manuscripts or writings and combining them into one book.
The two are totally different in doctrine as far as I know. The Vetus Latina carried on because most people of the time rejected the Latin Vulgate of Jerome and sticked with the Vetus Latina. This is said to be the source of bible translations such as the King James Bible, the original line started at the Vetus Latina.
Some people claim that the line of the KJV started with the Peshitta, i believe it started with the Vetus Latina. The earliest complete Peshitta is 5th Century where as many Vetus Latina Manuscripts are way older.
The Vetus Latina was written in new latin not in what is known as “old latin”.
If someone has better evidence than the above please comment.