I think the hammers are mostly still US made, just possibly using foreign made steel, which may be an availability thing, or a contract thing, since Marshalltown just acquired Vaughan recently, and may not have the contracts in place to source the proper steel alloys.
It’s also possible Vaughan ignored proper reporting on the sources of some materials,

, although I have no explicit knowledge backing this.
It’s possible some material that normally isn’t thought of, such as the powder coat finish, or the rubber handles use imported source materials.
I would presume Marshalltown, given what they already manufacture, has the resources to properly test steel, and can tell if imported steel has any issues.
With certain items like the Titanium hammers, I believe those were always made with imported heads.