If the computer system works perfectly, then the check-in agent in Vietnam would be the one indicating to you, less than 6 month validity on PP. Good Luck,,,best to leave 1 day before from Vietnam.
The Thai immigration information clearly states that you are supposed to have 6 months of validity left on your passport from the date of planned return date.
I have T-mobile Intl, and I don't use anything else,,,messenger/whatsapp/Line etc, unlimited data, and actual calls 25c,,but whoever needs that anymore,,
sounds like you are traveling on 2 different airlines and no luggage agreement between the 2? maybe a transfer counter at SGN when you arrive....and see if they can set up luggage to forwarded to 2nd flight carrier.