Are these vacations genuine? How can they possibly sell such an expensive package for such an incredibly low price?

Absolutely genuine! Nearly all hotels, resorts, cruise lines, etc. have vacancies on an ongoing basis. They find that participating in Vacation Package programs is a viable form of "advertising" to bring in guests who obviously then spend money in their restaurants, bars, shops, etc. Then, these trips have long been a mainstay for several agents. Today many savvy travelers use promotional packages such as this for their vacations savings thousands of dollars each year.