hy i messed up with vises / fixtures for bigger cutting forces, and i can tell that what you did is pretty good for some parts, while, if you clamp parts at your maximum vice capacity, than you just decreased it's holding rigidity; that solution requires some paralelism and 90 degress conditions on original vice + your profiles, that, even if are done at best accuracy, may lead to vibrations
what you did is perfect just keep it up if you are satisfied, than all is great