Actually a throttle valve may work if you need to fill a vessel up to a specific pressure and the regulators output pressure is to high you can slow the fill rate down and more accurately fill your application. you will need a separate pressure transducer downstream of the throttle valve probably about 0-100 torr. The fact that you need sub-atmospheric pressures makes me believe you are probably not dealing with a continuous flow device.
makes good products just don't use the brass ones!!