These rumors come up all over the place where Disney has supposedly bought property here, there and everywhere, if they were all true, Disney would own most of the country by now.
On the off-chance that it were true, it would not make sense to build a theme park in Vegas. Others have tried without success. A few years back, Vegas tried to become more family oriented and it just didn't work. Vegas does best with the 'what happens in Vegas, stays in Vegas' theme.
What could possibly make sense is using the land for a DVC resort. There have been rumors that DVC is once again looking to build resorts off of the WDW property. Every major timeshare operation has a presence in Vegas, if DVC were going to expand in a big way, they would almost have to do Vegas at some point.