Affordable housing - Vacant Building Credit versus NPPF para 64 - Public forum - Planning Advisory Service (PAS)

We have a brownfield redevelopment proposal where the floorspace is such that Vacant Building Credit removes the requirement for any affordable housing.  NPPF para 63 mentions the affordable housing requirement being "reduced by a proportionate amount". 

But NPPF para 64 asks for 10% of the homes to be available for affordable home ownership.  The paragraph lists four exemptions, none of which refer to VBC. 

I can't see any guidance in PPG on this scenario. 

Which takes precedence - NPPF 63 or 64?  Viability usually trumps affordable housing, but VBC is not all about viability...