If you only intend to machine aluminium,you should have bought a mill.I fear that you may be introducing difficulties by installing 12NM steppers as the machine was built to hold the workpiece in place with 8NM applied to the drive and you won't be able to increase atmospheric pressure by 50% as easily as you can bolt a new motor in place.You also need sufficient surface area to allow the workpiece to remain in place and a piece of wood occupying most of the table would be enough a small aluminium component or a machine vice wouldn't have the area.While you could bolt a machine vice in place,it would lose a proportion of your Z axis.

I have only used two machines with vacuum systems and only one of those had a pressure relief valve.It only operated if we turned off all the valves to the zones of the table.The other ,smaller machine had no valves to table zones and no pressure relief valve-we just turned the pump on or off with a push button switch.