I have been able to receive the one time free change benefit, it was showing up as if I purchased it. Also the priorty boarding benefit and fast track benefit. You need to add Vueling Visa card in your Vueling profile and subsequent transactions through App or online account will trigger these benefits. Other passengers on your itinerary will receive the benifit only if they are Vueling Up card holder and when you book the flight, you need to include their vueling membership account in the reservation.
But recently, I think it may due to system change, I no longer see these benefits included in the reservation confirmation. When I try to change a flight I booked last weekent, it is asking for 50 euro change fee.