Because they get cheaper tariffs on exports to the US, in the knowledge that they don’t want to import much at all from the US, so it makes Trump think he has something when he has nothing.

US goods will be cheaper in Vietnam compared to other countries in the region. US companies will be more likely to invest in Vietnam compared to other countries in the region. If 20% is low relative to where their regional competitors end up it's not a bad deal. I don't think a lot of US produced goods compete with local products so its not likely to cause domestic industry much problems.

I suspect they are “caving” a lot like Mexico and Canada “caved” earlier-the request made is already fulfilled.

Some countries are caving to US on war. Some on trade. VN was threatened with 46% tariffs. The 40% transhipment rate is something easily avoided. I doubt VN will import more from US. VN making a deal can still provide best value apparel to US

He's got an extensive history of lying about (among so many things) what so-and-so told him in a private meeting, only to be called out and contradicted by that very person the following day.

Throw another one on the pile, most likely.