VC sub-slot is automatically unlocked when a unit is level 99 or higher.
A Vision Card equipped in the sub-slot will only give a portion of the stats, party effects, and unit effects, as well as lowered cost.
All of the effects in the sub-slot will stack with the main slot, but if the effects are similar with another VC from another character, only the highest values will be applied.
If the 2 equipped VCs have abilities, both will be usable during battle. However, the ability of the VC equipped in the sub-slot will only be usable if the VC is level 99.
The amount of stats and effects obtained from the sub-VCs varies. For a lot of the powerful ones, only about 1/3 are obtained. Stats aren't always reduced through percentages since some of them can end up becoming +0 if used in the sub-slot.
Certain effects like JP increase up or Damage Limit up aren't reduced, while certain effects like Acquired AP up aren't reduced as much.