The Vietnamese must be living a clean life to get this kind of unexpected blessing:
Despite objections from some army commanders in southern China (especially on the Burmese and Vietnamese borders) the Chinese Army has retired the last of its WZ131 (Type 62) light tanks. This 21 ton vehicle was in service for fifty years and about 1,500 were produced between 1963 and 1989. Armed with an 85mm gun and three machine-guns (tw0 7.62mm and one 12.7mm) it had a four man crew and a top speed of 60 kilometers an hour. But the best aspect of the WZ131 was its mobility in swampy terrain, which there is a lot of in southern China. That means the WZ131 could accompany infantry into all but the worst (for vehicles) terrain.
The replacement vehicle is not capable of crossing the same terrain. So Vietnam's defensive problems are simplified, since the Chinese will be more restricted to attacking along the roads and whatever firm terrain there is.
Vietnam could still lose, of course. But their odds just went up.