Regardless of any billing add-on, your ability to get coverage abroad (which calls, texts & mobile data rely on) is dependent on roaming being activated - which allows your phone to connect to foreign networks.
On pay-monthly, this needs to be enabled manually - usually by texting ROAMING to 150 before leaving the UK.
The Roaming Pass is not valid in Vietnam, the charges are set out at https://ee.co.uk/help/help-new/roaming-costs
