Does a vacation on Nantucket in a 1829 salt box, or a secluded home on St John in the Virgin Islands sound appealing, but impossible to find? Hideaways, a company headquartered in Concord, Massachusetts, has started a clearing house service for vacation home rentals in New England, Florida and the Caribbean. This service custom matches vacationer's needs with more than 500 properties available for rent, ranging from secluded mountain cabins to sumptuous Caribbean villas. Prospective renters are given complete information and photographs of the property, then are refered directly to the property owner or agent to finalize arrangements. Vacationers are charged a fee of only $15 to $20 for a search, depending on the area. Vacation home owners, brokers, and condominium managers can subscribe to the service for a fee of $35 per property per year, with discounts for multiple listings.for further information, contact Hideaways, P.O. Box 1475, C oncord, MA 01742.