Right now, we are having highs in the 60's and lows in the 40's, which translates to somewhere around highs near 20 and lows at about 6 degrees celsuis. It can get fairly windy, especially at night. I would recommend bringing a jacket for nights and you should be fine with a sweater during the day. In all honesty, the wind can be a larger factor in what you need to wear than the actual temperature. I wouldn't plan on wearing short sleeves, but you won't need gloves or a hat either. The good news is that it doesn't look like we'll have rain any time soon.
Hope this helps; enjoy your time in Las Vegas.