Vegas gets like 90% of their water from Lake Mead... and although the levels have been dropping due to Arizona, Utah, Nevada and California pulling more from the Colorado River every year, thankfully there's other options for Vegas. Not that I care about the fucking raiders.
My guess is, Phoenix puts more pressure on the supply than Vegas. Ever been to Phoenix? Danged near every house has a pool. I ain't joking. You fly in over the neighborhoods, and 3 out of 4 homes have pools. So many, that in the heat of the summer, it gets fucking HUMID. In the driest desert in the US.
Vegas ain't going anywhere... but if AZ, CA, and NV are to get serious about water conservation, they'll be forced to reduce consumption one way or another so better to make those changes now so they aren't as drastic.