There's a scene in the novel where Vito, Tom, Clemenza, and Tessio get together to discuss the results of the big Dons meeting earlier in the day.

Vito says

Tom, I want you to put together a group of men who will go to Las Vegas and give me a full report on what is going on out there.....let's find out what really can be done out there.

So I'd say that initially the interest in Vegas was the Don's.