Three Little Reasons

France wanted to invade Vietnam because of a few reasons:

1.  French wanted to protect the missionaries that were being attacked by the Vietnamese government. Tu Dac, the Vietnamese emperor at the time, attempted to eliminate all Christians from the mainly Buddhist country. [D] 
2. During a time of imperialism, the French did not want to be left behind in the race of global power. Everyone wanted a piece of foreign territory for raw materials and new markets. Vietnam was a direct trade center with China and would expand territory for French capitalism. As Napoleon’s specially appointed committee (to provide rationale behind the invasion) pointed out: “will we be the only ones without possessions in Asia, where the English, Dutch, Spanish, and even Russians are strengthening their positions?” [D] Napoleon also had an “obsession with national glory” [D]. Since England was establishing herself in Asia, why shouldn't France as well? 
3. France was also interested in Vietnam for its trade with China. With raw materials of coffee, rubber, and tea, Vietnam was a place for imports and exports of goods. [E]

All of these reasons to Napoleon's invasion. Forward march the French army! Huzzah!