That is a tricky claim to prove reasonable or not. However, it is sort of like saying that Austria-Hungary and Serbia was responsible for WWII: Yes, its actions did spark WWI which led to the Versailles Treaty which had a tremendous effect on the German population and radicalized some parts of German society which ultimately led to Hitler and the start of WWII.

However, France is not one of the root causes of the Vietnam War. Just as much as Austria-Hungary and Serbia is not a root cause of World War II.

It is reasonable to say that the First Indochina War, which led to the partition of Vietnam at the 17th parallel is one of the root causes. However, this has to be put into a context and be explained. Between 1946 and 1950, the United States looked at the Indochina conflict with disinterest. The war seemed to be one of decolonization and rather insignificant with the more pressing conflicts around the globe against communism. However, by 1950, there was a turn in US policy and the United States suddenly began to pump in money, equipment and weapons to the French expeditionary army. The reason for this was China. With the creation of the People's Republic of China in 1949 and its interest in supporting the Viet Minh, the war suddenly found itself thrown away from the localized conflict of decolonization that some saw it as and put into the larger context of the Cold War. Now the war , in the yes of the US, was about the spread of communism and as China began to pump in military advisors, equipment and weapons - the United States matched that with the French.

We can very clearly see that the French left for good in 1955 at which time the United States took over the supporting role for the newly created Republic of South Vietnam. France does not play much of a role after that and if we look at the causes that led to the Vietnam War (looking at the period of 1959 and 1965, prior to the American War), we see that none of those were really influenced by previous French actions. Instead, one of the root causes of the Vietnam War can be found with the actions of China and the United States in 1950 and the choice of the US to take over from the French in 1955 and support the RVN in defending itself against a possible North Vietnamese invasion (and later on, against a nascent insurgency which turned into a full war).