You'll be fine, but pack for cold weather (down around zero) as well as for warm weather. In Vegas, expect highs in the mid-teens and lows around freezing, but it's very unlikely you'll get any colder than that (and you'll be inside for most of it anyway). Bring a warm jacket of some sort—alternatively a sweatshirt with a couple of layers underneath—and wear jeans or long pants.
LA will be a lot milder; it's not going to get below freezing. More likely you'll have highs around 20 degrees Celsius and lows around 5; you'll probably still want that warm jacket in the mornings, though.