Diesels are different than gas engines. The vacuum reference port is meant to regulate more fuel back to the tank at idle on a gas engine when vacuum is present, then give the rails more fuel when accelerating and vacuum diminishes. On a diesel, there is no natural vacuum at idle. Ford provided a belt driven vacuum pump and regulator for the Powerstroke so as to handle things that need vacuum like the brake booster and heater controls. This pump set up has the same vacuum throughout the rpm range, so there is no benefit for the vacuum boost reference port on an aftermarket fuel pressure regulator. Now some regulators can also reference turbo boost which could benefit fuel pressure under acceleration, but I don't think it would have significant adjustability, because the regulator would be closing off too much fuel return to the tanks long before full turbo boost came on. Also, you'd be introducing air in very close proximity to the fuel line system, which could have its' drawbacks as well.