What's funny is that the ROC actually claims more land than the PRC.
The funnier thing is that the ROC is essentially forced to continue claiming those lands lest it do anything constituted as a declaration of independence.
But China and Vietnam have never been particularly friendly, Hanoi was always more in the Soviet sphere and nowadays in the American sphere- making the Vietnam War the second greatest geopolitical blunder of the second half of the 20th century (not to mention the fact Ho Chi Minh wanted US aid from the get go).
Now it remains to be seen what support, if any, outside powers (read Moscow and Washington) shall lend Vietnam, and whether it shall be merely diplomatic or take a more active role.
And, as said, the idea China is not some imperialist nation by default is silly. Traditionally, the Chinese view of the nation was quite different from our modern Westphalian convention. Almost every state between the Andaman Sea, the Kra Isthmus, and the Sea of Japan was made to pay tribute to the Heavenly Kingdom at one point or another. They mightn't have conquered them outright, but they made their presence felt. This attitude, in some ways, persists to this day.
