From USA, to Vietnam, it will cost approxiamately .75. After May 14, it will cost .77. If your card is a large one, or is a long one, it will cost .84 before May 14, .86 after May 14. This is the basis for a .75 postcard:
Height: 3 1/2 inches to 4 1/4 inches
Length: 5 inches to 6 inches
A large or oversize postcard is when the:
Height exceeds 6-1/8 inches, or
Length exceeds 11-1/2 inches, or
Thickness exceeds 1/4 inch, or
Length divided by height is less than 1.3 or more than 2.5.