There are no federal law restrictions.
Laws vary from state to state.
I have assorted vest up to level4a.
I was on a tactical team for 10 years.
Some were issued, paid for by fed grant, and registered to me. So, they were mine and went with me when i retired.
I did buy two i think, they came from gall's, it was a private sale not through the dept. No questions were asked.
Just because you can order something does not mean it is legal to posses where you are. Check you state laws.
In this state, if you commit a felony while wearing a vest their is a sentence enhancement of about 10 years.