Doesn't look like you can. Tech specs only list the 560X as an upgrade option for the base 2.2 model. Clicking on "select" for the base model shows only the 560X upgrade option as well. Clicking on "select" for the upper tier model lists "Graphics - New options coming late November" and both Pro Vega options listed below that.
Well cross my fingers it doesn't overheat then I guess. Still for how much Apple likes to put emphasis on quiet machines you'd think they'd give the 2.2 model a Vega option so I can have a beast machine that still stays as quiet and cool as possible.