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.
The airline will most likely not check you in as you don't have a return ticket and not a valid visa. Information in the systems clearly advises pax to have a return ticket.