Basically, foreigners are like walking ATMs to them. One thing that vietnamese are damm good at, is that they use the soft approach. They will tell their daughters how pitiful they are or how pitiful their siblings are, so after listening to the VIETNAMESE SAD STORY, you have to pay the cinema ticket la. But this cinema ticket is not cheap leh.....
The person that needs to have a strong will is your wife. This is very important. Talk to her slowly, let her see that her siblings have all grown up and they can earn their living. Let her know that her parents doesn't need a lot of money to survive in the province. If your wife understands these, your life will be very good and you need not worry about sending them money.