Ahh the lovely Las Vegas sign. I got married just up the street from this sign at the Little Church of the West.
Sadly alarnold, when dealing with popular spots like the Las Vegas sign, you have to make some sacrifices or deal with it. Wind is one thing, crowds are another. It may be a hassle lugging a soft-box and some sandbags to that location, but in reality, parking is just down the street. Setting up the soft-box and other lighting may have actually helped you in this case. I would have told the people that this was for a magazine spread or something like that, they they might have been a little more helpful in getting out of the way, maybe.
Vegas never sleeps, but people do, If the above dod not work I would have scheduled it for 2-4 am shot when most of the tourists are either too drunk to get out to the sign or dropping major coinage in the slots.
It would be the same for any major attraction, you either shoot with the people in it or shoot when there is no one there. For instance, I shot the Walt Disney Concert Hall at midnight. Why? Because it is right smack in the middle of downtown LA and at midnight the only people out are drunks and police, perfect for shooting, in a photography way.
As for security, losing a $20 bill in their hands doesn’t hurt. It’s not a bribe, if they found what you dropped in front of them, get it.
This falls under my Know Your Surroundings from the above post. It is easy for anyone to get a shot when the conditions are in your favor, but knowing how to work with un-favorable conditions and making it work will set you apart. It’s not going to be easy, mind you, it never is.