The dress code at most Vegas clubs is fashionably casual. No shorts, no tank tops, no flip flops, no overly baggy t-shirts, no hats, no basketball or workout sneakers, and no athletic gear (jerseys, sweats, etc.). Everything else is pretty much OK. Guys can wear rubber-soled casual shoes - Vans, Skechers, etc. - as long as they are NOT workout shoes. Collared shirts and button-downs are encouraged but NOT required - a form-fitting stylish T-shirt will work just fine, and even better if you throw a blazer or sport coat on top if it.
That being said, most people do dress up to the nines in Vegas, especially for the hotter high-end nightclubs with elevated price points (Omnia, XS, Hakkasan, Marquee). You will NOT feel out of place if you decide to don slacks, a blazer, and dress shoes, or even a well-tailored expensive suit. Ladies often wear revealing dresses with high heels and even cocktail dresses and evening gowns - you can never be too dressed up in Sin City. And, every so often you'll see a wedding party out and about- with guys wearing tuxedos and girls wearing bridesmaids dresses - and even these groups don't look out of place.
When partying late into the night in Vegas, you should always err on the side of caution - it's better to be dressed up than dressed down for your chances of getting in with no hassle.