Cover shoulders and knees when entering pagodas like Trấn Quốc or Một Cột. Bring a scarf or light cover-up.
Some temples lend wrap skirts at the entrance, but don't count on it. Remove hats and sunglasses inside the main hall.
Don't turn your back on the Buddha when leaving — step backwards a few paces, then turn. Locals do this naturally; mirroring shows respect.