Policy provided additional insured with liability coverage only with respect to its vicarious liability arising out of subcontractor/named insured's acts or omissions. Underlying suit contained allegations of both direct liability and vicarious liability, so insurer had duty to defend general contractor. Court rejected insurer's argument that there could be no duty to defend because Illinois law would not impose imputed liability, but duty to defend applies even to hopeless lawsuits, whether they are unfounded, false or fraudulent.