That's not how you would want to measure this, as it doesn't much matter whether a good is directly exported from China to the US, or whether it is routed through a third country like Vietnam, or used as an intermediate input to a different good which is then exported to the US, etc.

What you should take note of, is that China's trade surplus tracks the US trade deficit extremely closely, and this has been the case for the last 20 years.

Said another way, most nations in the world run roughly balanced trade. There is only one nation on earth that can sustain large and prolonged trade deficits. Only one. That is the US. So if you are a nation whose entire economic strategy rests on running large and sustained trade surpluses, then you are going to be exporting your surplus to the US, directly or indirectly, and only to the US.

Now in practice, some of that will be direct sales to the US, some will be indirect sales to the US, but at the end of the day, if you want to run a 1.2 trillion dollar trade surplus, then you have to hope to God that the US is willing to run at least a trillion dollar trade deficit, because no other country is going to do that for you, the entire rest of the world may be willing to accomodate a hundred billion or so, but for everything else, you will depend on the US, directly or indirectly, to absorb your surplus.