VTG is one who does a lot of deceptive advertising. I can tell you for sure the rates showing are probably the ones not combinable with the $150 OBC or you could book it cheaper other ways. .. "the promotion may not be combinable" is your hint, to dig deeper on the price. Maybe check travelocity, because you can click thru it and see what the sale price actually is to get that OBC.
Book by July 31, 2011 and receive a FREE $150 per cabin onboard credit on select categories! Promotions may not be combinable with all fares.