I can think of a hundred ways to approach this topic, most of them pretty funny. I think I'll save the humor for now.
The time is right for this. Historically the Vietnamese have stung the Chinese the several times the two countries have fought. So from a psych-war standpoint it's a smart move. And it helps tighten the strategic encirclement of the PRC, a plan that's been in the works for some years.
Plus you've got the Mad Dog diplobroing in Hanoi. If I were the Chinese I'd be so concerned the yellow river would be flowing in my pants.