i have a book called "afoot & afield in Las Vegas & southern Nevada" that i have used to help plan trips in the Vegas area (i live about 5 hours south of LV and get up there occasionally).
yes, the nights can get very cold in the high desert, but the lower your elevation the warmer the nights will be. still, be prepared for potentially freezing weather. freak snow flurries have happened in the high deserts, but you'll never get "snowed in."