Last winter I never broke out a real jacket. I survived the whole winter with a sweatshirt. Year prior it did get cold enough to force me to break out something a little thicker with a bit more wind protection. It's rare for the temp to drop below freezing except for a couple hours before dawn and it's usually into the 50's by mid-afternoon.
Vegas in January is usually pleasant. It ain't Florida, but it ain't North Dakota, either.
http://www.cdc.noaa.gov/cgi-bin/USclimat...gifields=itypea