[Pearls Before Swine]

Why would Viet Nam attack China?

I think "counter-attack" is the appropriate word. A more interesting question is "Why would the Russians sell those missiles to Vietnam?"

Also, unless the missiles are nuclear tipped (shudder), they won't make much difference.

[Army Air Corps]

Viet Nam has tended to have closer relationship with the Russians going back to Ho Chi Minh and his time spent in the USSR.