If you intend to live overseas with her for the entire process, get married and file the I-130. One immediate advantage is that the I-130 can be filed online.
Be aware that it will take about 18-24 months for the entire process. You would need to sort out your authorized stay situation in Vietnam.
Also, your income may change with you living overseas If you do earn enough, you will need to also prove the income will continue after you return to the US. But this is something to work on later. The I-864 isn't required until the NVC stage, which will be about a year from now.