Does VOC-free paint require VOC-free undercoat?
VOC solvents are used in paints because they are volatile; they will readily evaporate at room temperature.  When someone wants low-VOC paint, it means that they don't want (for health or environmental reasons) to release VOCs as the paint dries.  Since the undercoat would need to dry before the finish coat is applied, using an undercoat that is not low-VOC would defeat the purpose.