dont worry about the vietnamese phone cards,

go to an ALLPHONES and ask for a lebara sim card ( a new carrier with extremely cheap iternational and local calling costs)

charges are as follows for vietnam

Vietnam
.................................................. ...
Mobile $ 0.05 /min

Landline $ 0.05 /min


credit you can purchase
$10
$20
$30
etc

and it lasts up to 3 months

put this sim into any unlocked phone and away you go.

Regards
Fil