Vegas could support an NFL franchise but I question the ability of the team to attract local fans. It'll be another strip attraction like Celine Dion, Blue Man Group, etc. You will get people in Vegas for the weekend but how many local fans are you going to attract? It's a transient town that sees a lot of people moving in and out.
Until a domed stadium is built, I really question their ability to attract people for preseason and early regular season games... having visited in August when the temperature was 110, you'd have to pay me large sums of money to sit out in the stands in the sun to watch a game. The preseason games could be played at night, but unless they plan on playing the first few weeks of the regular season in primetime when it begins to cool off, I think it's going to be a really hard sell.