Hi. This is proving to be too complicated. It will be easier to kill the original supply, and just use the original switch to manually turn-on the vacuum. I guess it’s possible to use some trigger from the SG-4 or MG, to add a relay etc but it becomes way above my expertise…The LM317 should be OK at 1.5A. The SG4 draws minimum power and the MA3D is class D so it is 90-95% efficient.