Most Vietnamese homes, many temples, and some traditional restaurants expect shoes off at the door. Look for a pile of shoes as your cue.
Wear slip-ons or sandals to make this painless. Socks should be clean and hole-free, as you'll be very visible.
In temples, walk barefoot or in socks past the threshold. Never point your feet at a Buddha statue or an altar — sit cross-legged or tuck your feet behind you.