If Vegas will not allow it, there's no option but to choose a different venue. Besides, the biggest revenue of this fight might be coming from the PPV, so the venue is not a big deal, the most important is that they will make the fight entertaining so people will get the value of their money.
more on PPV but maybe they are also trying to convinced gamblers to watch and gamble in this exhibition match, not sure though if that's the real score behind in choosing vegas as venue, but it's true, if vegas is not available the promoters can still find alternative venue to continue this deal, if they are trying to hook the attention of more fans, providing details with any possibilities will surely trigger that interest.
Else, the talks about this possible fight will die and there would be no interest that will come from any venues of social medias.
They need to act and find the suited way to promote and to make this fight happen.