Reminds me of the old rumor that the city of Usa, Japan exists so they can print "Made in Usa" on exports. They actually don't do that and Usa has held its name longer than the actual United Stats has existed but it still makes for a good story.

How would they be risking it?

Who is incentivized in this system to say that this product is actually coming from China? Is it the US government regulators who just lost their jobs? Or the Businesses themselves who don't want to pay higher tariffs? Is it China who avoids the tariff? Is it Vietnam who just got paid for doing nothing?

The answer would typically be that the US gov't would monitor this. But that is not a muscle this administration is capable of. That would require nuance and strategy, and it's very clear from this move that this administration doesn't have that capability.