Its nice to have a rich gf, but no matter how rich she is, the wealth still belongs to her family. As an asian man, its better to make the money by yourself. Rich vietnamese normally will prefer their daughters to marry local viet man from about the same family status. Marrying foreign man are deem as the doings of poor provincial farmers who wants to `sell' their daughters to make some money. To the rich viet, such act is deem degrading and unreasonable.
No matter what, it takes time to know a person.