The fact that any new regulation wouldn't have prevent X mass shooting or Y mass shooting is completely irrelevant. Either it makes sense to exempt gun shows and private purchases of firearms from background checks or it doesn't? Most people support universal background checks and the fact that it wouldn't have prevented Vegas doesn't change that. Unless you want to ensure easier access to guns for convicted felons or terrorists, there is absolutely no defense of the gun show loophole.  

If a background check prevents a convicted felon with ill intent from purchasing a firearm and that felon isn't able to shoot up a liquor store, shopping mall or whatever that day, then how do you measure something that doesn't happen? Its like having to prove a negative. 

Whether or not they would have prevented this mass shooting or that one, universal background checks are sensible. Maintaining gun show and private purchase exemptions aren't.