China has been far more worried about (a) internal security, the party is deathly afraid of its own people, and (b) its border countries. On the latter point, China and Russia have even fought some skirmishes over their vast border, and have only been recently back on friendly terms.

China did make it all the way to Hanoi during the Sino-Vietnam war back in 1979. I'm not sure what definition of occupy you want to use here.