One other point to know about lounge access for Vietnam domestic flights: If you have an economy ticket and buy up to first class when checking in, my experience is you will not get lounge access. You only get the inflight benefits (first class seat and meal service), you don't get the ground benefits. If you're ST Elite/Elite Plus, you get access to the VIP security screening (even if in economy IIRC), which is actually quite helpful -- the line is usually very short, and often pilots and flight attendants outnumber actual passengers.