Vatican City has several consecrated spaces and there is a dress code required to access them. While you do not need to dress up to go to the Vatican, the general rules is to dress conservatively: opt for covered shoulders (men and women), long trousers, below the knee skirts and avoid showing off cleavage or bare backs. You can wear sandals and sneakers: I do however recommend you do not wear flip flops, mostly for the safety of your feet in such a crowded space!