vztube.js

var domains = ['vztube.verizon.com', 'vztube.vzwcorp.com', 'vztube.verizonwireless.com'];

function getFirstAccessibleDomain(callback) {
  var domain = domains.shift();
  var image = document.createElement('img');
  image.onload = function(){ callback(null, domain); };
  image.onerror = function(){ 
    if (domains.length) {
      getFirstAccessibleDomain(callback);
    } else {
      callback(new Error('No domains acessible.'), null); 
    }
  };
  image.src = 'https://' + domain + '/images/spacer.gif?cache=' + (+new Date()) + '&referrer=' + encodeURIComponent(window.location.href);
}

getFirstAccessibleDomain(function(err, domain){
  console.log(err, domain);
});