port your number to Google Voice.
install Google Hangouts on your phone. Google will manage your incoming texts and calls. you can also make free outgoing texts/calls to the US.
get a local prepaid SIM (must include data) and pop it into that phone.
install Skype in the phone. set up Skype Caller-ID. it takes 48 hours to activate.
after activation, you can now send OUTGOING texts to non-US numbers (requires Skype credit).
this is because your local VN prepaid number expires after 1-3 month if not topped up (depending on plan)… if you top it up religiously, that’s okay. as i fly in/out of VN, my number some times expires and i have to get a new one.
thus i prefer to give out my US-number and communicate via WhatsApp with the Vietnamese. i save my local VN prepaid minutes for local VN landline calls.
if you have a cloud-based Office 365 subscription, that will add 60 world minutes/month (excluding Vietnam) to your Skype account (linking/activation required).
this gives you a back-up service if Hangouts doesn’t work well… as well as for making calls to non-US and non-VN numbers (check list of eligible countries for Office 365 Skype World Minutes to be sure).
roam on your old number. set up a new GV number instead of porting your old one, and follow as above.