Christian asked if there was an aerodynamicist in the house - I guess that would be me!
The original question was whether there was any vortex activity in subsonic cruise, but the discussion went on to ask about designing for subsonic drag I think.
The answer to the first bit is that the vortex flow started in a gentle manner from about 6 or 7 deg AoA and got steadily stronger. Depending on the chosen cruise speed and the aircraft weight, the subsonic cruise AoA would have been in the region of 4.5 to 5 degrees, i.e. below any significant vortex development. 6/7 deg would correspond to something in the range 250 to 280 kts probably (I haven't done the sums)
What we were trying to do for subsonic cruise was to have what is known in the trade as 'leading edge suction' acting on a nice bit of forward facing area so that it tried to drag the aircraft forwards as it were. As you can see from the diagram the prototype aircraft had a much more cambered LE so that both suction and forward facing area were very reasonable. This prototype shape was nicely rounded so that LE separation and top surface vortex generation started at a higher AoA than on the production aircraft. Unfortunately this shape, which featured a rather sharp LE on the undersurface, generated a vortex on the undersurface of the wing in supersonic flight and low AoA (near zero 'g'). This vortex got into the intake and caused the engine to surge, so we had to redesign the LE ahead of the intakes as shown. This cost us a little subsonic drag, so you can see from the diagram what you need to do to keep subsonic cruise drag down.
Hope this answers the questions
CliveL