At sidewalk eateries, a 'cô' (auntie) will bring complimentary jasmine iced tea (trà đá). It's free; don't refuse, just sip.
Refilling is automatic. If you don't want more, leave the glass slightly full as a signal.
This tea is brewed fresh and safe. The ice is industrial-machine ice (cylindrical, hollow) — also safe at any reputable stall.