Heard that as well about the possible armor. In my research purely for educational purposes a good level 3 would be fairly obvious to see. I know there are regular looking shirts, but unsure if that would be effective against that round. Here is a good example of something discreet. I think there would be an outline for sure. I am by no means an expert so i could be wrong on the discreet options, especially given the financial resources at hand.
https://bulletproofeveryone.com/the-operator-2-0-undershirt/