The newly formed subsidiary is United Vacations, based in Marina del Rey. Mike Corbett, president of the subsidiary, said the tours will include flights on United, car rentals, hotels and airport transportation. Sales will begin in April, only in California and only through travel agents. Start-up costs, Corbett said, were about $1.5 million. United carries 3.6 million travelers annually between the mainland and Hawaii, about 50% of the market.