Seems like a solution seeking a problem. Having the soft armor attached to the shirt isn't going to help conceal it, and also means it might not be where you want it if you buy any size other than really snug. Wearing a soft vest under a regular loose T shirt would be better.
But that said, I suspect most civilians will never have to conceal armor to require this in the first place. Chances are, if the armor is required, concealment is no longer an issue. Or at least I don't think it would matter while you're most likely also openly holding a rifle and open carrying a side arm as well. But maybe there are scenarios I'm not thinking of.