Well, in fairness to them VPI are now back in the game, with possibly plausible excuses for the delay.

I may have misunderstood their explanation before - I believe the fault is that the shaft is slightly too wide at the bearing end for the rim drive.  Of a number of potential solutions posited by VPI I have opted for a replacement shaft, with a slightly narrowed end, upon which the Rim Drive will sit.

Once I've received the part and installed everything, I'll post my impressions!