The spousal visa is certainly superior, and since you are currently together, you could do the Utah Zoom wedding and file the I130 as soon as you get the marriage certificate. There is no real time advantage to the I129F anymore. As to the I864, I would go through the form, understand your household size and determine if you meet the requirements. You can stay with her as long as you are doing it legally, but keep in mind the spousal visa processing can be 18-24 months. As to changing your address, that depends on your situation, but if you are staying in Vietnam until your future spouse has her interview, you will need to provide proof you have re-established domicile back in the US.
Good Luck!