Some machines in the metro stations have the option to put credit on your Viva Viagem ticket using a credit card slot. MultiBanco is written on the slot where you need to insert the credit card. Will it accept any other credit card, or just MultiBanco?
According to the details on Metro Lisboa, credit cards can be used at ticket offices and automatic ticket vending machines. However, in some stations, tickets can only purchased at automatic vending machines and, as noted, only accept the Multibanco, a Portuguese debit card.
Multibanco is the single unified interbank network in Portugal, linking ATMs of all Portuguese banks, so some debit cards may work.
Although inconvenient, you should be able to find a nearby ATM (probably Multibanco), where you can get Euros to use in the ticket vending machine.