Although Las Vegas casinos were filled with people dressed to the nines during the Rat Pack era, things have gotten much more casual over the past couple of decades. The warm weather and informal character of Las Vegas render casual clothing appropriate day and night. However, there are some exceptions. A small number of restaurants require jackets for men, and some of the city's increasingly exclusive and overhyped "ultralounges" and high-profile dance clubs have specific requirements, such as no sneakers or jeans, or that you must wear dark shoes or collared shirts. At a minimum, even if there's no set dress code, you're going to fit in with the scene if you make some effort to dress stylishly when heading out either to the hipper nightclubs or even trendier restaurants (i.e., those helmed by celeb chefs or with trendy followings and cool decor). Just as an example, where jeans and T-shirts might be technically allowed at some establishments, try to wear plain, fitted T-shirts versus those with logos and designs, and choose jeans that are appropriate for a venue (crisp and clean for a nice restaurant, designer labels for a top club). It's always good to pack a few stylish outfits for the evening, and when you're making dinner reservations at an upscale spot or considering a visit to a nightclub, ask for the dress-code specifics. Also, flip-flops are best kept to pool areas.
Although the desert sun keeps temperatures scorching outside in warmer months, the casinos are ice-cold. Your best insurance is to dress in layers. The blasting air-conditioning may feel good at first, but if you plan to spend some time inside, bring a light sweater or jacket in case you feel chilly.
Always wear comfortable shoes; no matter what your intentions, you cover a lot of ground on foot.