So long as I asked the agents the right questions, it seems that you can now use VX as well as AS for the domestic connection in conjunction with a CX award. Note 5 of the T&C suggests that VX/F can't be booked on some routes ("Not valid for travel in Business/First on nonstop flights between New York - JFK and Los Angeles, San Francisco, or Seattle.") I'm going to email them to confirm.