If the Vodafone shop at arrivals is busy (they often are when long haul flights arrive), try their shop up in the departures. If even that is busy, just around from the Vodafone departures shop is a Correios (Post office) and just outside the Correios is a MEO (a Vodafone competitor) vending machine that sells SIM cards for similar prices. The MEO vending machine operates 24h a day, the Vodafone shops I think are 8am-10pm.