Verified answer

It wasn't a policy - it happened in stages, and like a lot of history, there wasn't really a plan that guided the whole thing.

The two main motivations were religious mission and trade. French priests got a foothold in the south in the late 1700s and established a presence that facilitated trade and other French interest in region. Over time there were elements of political instability and fears of other European influence that prompted them to take the whole of Vietnam.

But the real focus was China -- Vietnam actually didnt have much to trade. The hope was that the Mekong or Red River would provide a French-controlled highway into China (like British influence over the Yangtse region). In fact, neither river is navigable that far, so, whoops!

It was only later that the big export commodity of rubber became important--after they already had the protectorate over the region.