Hi Sanah,
While the dress code is quite relaxed in the major cities of Vietnam, its always better to dress modestly. While visiting the temples and pagodas, regardless of the fact that they have no strict regulations, to respect their customs, avoid anything overtly revealing or ostentatious. If you are visiting anytime between August to November, carry plenty of light t-shirts, jeans and cotton wear. Another essential if your are travelling during this season will be raincoats, as Vietnam sees an abundance of downpour during these months.