Vietnam has suddenly turned to France to refurbish its military and as a source of new military equipment. This was somewhat surprising. Last December, Vietnam signed a contract to purchase six Kilo class submarines, for $300 million each, from Russia. This was part of a larger package, that included over a dozen patrol boats, at least two frigates and dozens of aircraft. In effect, Russia was supplying weapons to modernize Vietnam's armed forces. But the Vietnamese have rethought that decision.
Until Vietnam enacted market economy reforms in the last two decades, the country was broke. But that has changed, and Russia offered attractive prices. This annoyed China, which considers Vietnam part of southern China. But despite centuries of military efforts, the Chinese could never keep Vietnam under control. Now Russia is arming this wayward part of the motherland. China has not made open claims on Vietnam for over a century, but the animosity, and memories, are still there. And China was not happy about Russia stepping into this family feud.
Although Vietnam fought a bloody war with France from 1945-54, it was France that unified Vietnam in the 19th century (and turned the region into a French colony) and introduced modern technology, Western education and French culture in general. The Vietnamese have not forgotten, but there are no grudges. It's easy to be forgiving if you were the winner. The French are pragmatic, as military exports are a major chunk of their economy.
But how did France suddenly replace Russia as the major military supplier? There were several reasons, the major one being the recognition that Western military equipment and doctrine was superior to what the Chinese and Russians were selling. There was also the realization that Russia and China were getting pretty cozy, and that Russia and China also had a centuries old real estate dispute over who should really own Russia's far eastern provinces. If China wanted to renew old disputes with Vietnam, Russia would not be much help. China only has to bring up those old territorial claims, and Russia backs away from helping Vietnam in any way.
France, on the other hand, has no claims on Vietnam, and wants to keep China from becoming too much of a troublemaker. Vietnam has also been getting cozy with the United States, another old foe who also shares concerns about Chinese ambitions.