You make it sound like as if Vietnam and US have no war experience and China is the greatest military power to grace this world?
1. Overrunning Vietnam is not so easy through the ground because of its terrain which does not favours armoured thrusts.
2. US knows how to protect its military bases, even if the China miraculously reaches them.
You make it sound as if Vietnam isn't one of the poorest countries in Asia with the poorest equipped and trained troops. Vietnam's military is armed with decades-old technology. How long do you expect them to hold up against China's modern military? I say two weeks at the most.
How will the U.S. protect its bases against the overwhelming ballistic, cruise, and MLRS missile power of China? Magic? You do know the U.S. is moving its troops from Japan to Guam and getting the hell away from Chinese SRBMs, right? It doesn't solve the problem posed by China's IRBM (Mach 10), but at least the U.S. won't be staring at thousands of SRBMs (Mach 6).
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Here's a partial list of China's Military Technology. You tell me if China has an advantage over Vietnam.
China's Military Technological Milestones - Timeline
210 B.C. (2,200 years ago): China invents chrome-plating technology during Qin Dynasty under emperor Qin Shihuang.
1964: China detonated a 22-kiloton atomic bomb during the Mao Zedong administration.
1967: China detonated a 3.3-megaton thermonuclear bomb that was designed with abacus calculations.
1971: China launched its first DF-5 ICBM.
1984: China built its first cryogenic YF-73 rocket engine.
1995: China test-detonates kiloton Neutron Bomb
1998: Chinese J-10 Vigorous Dragon had its first flight. Officially unveiled in 2007.
2000: Chinese JSTARS Tu-154M/D Electronic Intelligence Aircraft in service (e.g. Careless B-4138).
2001: Chinese Type 99 Main Battle Tank in service.
2002: China's Type 093 Shang-class nuclear attack submarine (SSN) is launched.
2003: China sends its first taikonaut Yang Liwei into space.
2003: China's KJ-2000 AWACS with domestic AESA radar has its first flight.
2005: China's Type 052C Lanzhou-class AESA-equipped destroyer entered service.
2006: China's WS-10A turbofan engine certified for production.
2007: China clones world's first rabbit.
2007: Chinese direct-ascent ASAT shoots down orbiting satellite.
2007: Chinese DF-31A MIRVed ICBM in service.
2008: China conducts its first spacewalk with taikonaut Zhai Zhigang.
2010: China builds world's-fastest supercomputer Tianhe-1A.
2010: Chinese GBI (i.e. ground based interceptor) shoots down a ballistic missile during mid-course phase.
2010: Chinese WZ-10 Attack Helicopter in service.
2010: Chinese Type 094 Jin-class nuclear missile ballistic submarine (SSBN) in service.
2011: China unveiled the J-20 Mighty Dragon stealth superfighter.