My wife and I got married in Vegas in mid May. 107 degrees day of. We live here so we're used to it and it wasn't a problem. Guests from Oregon on the other hand.
If you're getting married outdoors the wind picks up in March through early April. Might not be a big deal down on the strip surrounded by the towers. March is also a busy time in Vegas due to March Madness.
Best weather in my opinion is April and late September/early October. So for your timeline I would shoot for April 15th - May 5th. Usually in the 80's, creeping into the low 90's by early May.
Great choice for a hotel. Cosmo is one of my favs.
