As I understand it,
Vc is the "Design Maximum Cruising Speed" which is a speed used during testing and certification to assess the structural strength of the aeroplane. The airoplane must be able to withstand vertical gusts of +/- 50 fps (at 20,000'). If flying faster than Vc for example in a shallow dive, then the aircraft only needs to witstand +/-25 fps gusts.
A safety factor is applied to this in the same way as an aerobatic aircraft may be permitted to say 6g with a wing which is known to fail at 10g. You wouldn't want to manouever at 10g if you knew the wings would fold at 10.1g! Same thing with the speeds.
Vc is the design speed which then has a safety factor applied to give Vmo/Mmo.
At least that's how I understand it,
Obs