Your guess is as good as any. I have been to vegas in September and encountered 107 degree hot temps. On the other hand I visited another time and the temps were in the 90's. You are at the mercy of the weather patterns at the time. You may want look up vegas weather prior to your trip to see how the weather and temps will be when you arrive there.
It was 100+ in early September last year when I was there. By late September the weather is usually pretty nice.
Accuweather or Weather.com historical monthly averages are good information and usually about right, but if you watch historical record highs and lows you'll see what others have said here-September weather is wildly inconsistent-it can be August hot or October cool. Historical averages say a high of 92 degrees and a low of 66-69 degrees. We're coming September 30. I'll probably bring a windbreaker, but I doubt I'll wear it. If you live anywhere North of St. Louis, think midsummer hot. September is not "autumn" in the desert-it's the last hot transition month to bearable temperatures. http://www.accuweather.com/en/us/las-vegas-nv/89101/month/329506?monyr=9/01/2016 https://weather.com/weather/monthly/l/USNV0049:1:US
I would certainly take what JACKPOT JOHNNY just posted over anything the average poster might give you. We always check one of the online services for historical and watch the updated forecasts. Anything else posted is just a wild opinion or guess.
We always travel to Vegas in September, I think the weather is pretty nice at that time of year. I would agree with previous posters about looking up historical data.
we consider September weather in LV to be great pool weather and plan to spend a couple of hours per day at the pool to manage the bankroll
Over three trips in Late September, the temps were 95 plus making it uncomfortable in the direct sun.
Late September , early October is one of the best times to travel to Vegas. It's still pool weather but the extremes of July and August are usually passed. I say usually, there are exceptions of course.
We go in late sept for the past 8or more years. Weather is great-80's. We were there 3rd week last year and it was beautiful. Going to be there on the 26th for 5 nights. Will be crazy downtown 9/23- 9/25 for Life is Beautiful concert