On days when I am feeling extra peppy I wear the armor under the HPG bag, but you get the idea how the HPG bag would work on its own. They make an elastic runner strap that clips to the bottom of the bag and around your chest to keep it secure. View attachment 659578
That is their largest pack, sitting on top of kydex mag pouches, on top of steel plates, on top of padding.
Without the vest it fits very snug, and the smaller packs would be even more comfortable.
Its not the best setup but I am still trying to figure out a good way to carry concealed with the vest.