If you want to save money and will go anywhere, try a vacation clearinghouse

Last year Dr. and Mrs. Dean Sommerfield were ready for a week-long vacation. They knew they wanted warm weather. And they knew what week they wanted it. But they had no idea of their destination until a couple of weeks before the trip.

A case of indecision? No, just budget-consciousness. A few weeks before their vacation, the Sommerfields called a clearinghouse to find out what package tours were available. They chose Puerto Vallarta, Mexico - and saved a bundle.

''It was a real nice deal,'' Dr. Sommerfield recalls. The price was $500 per person, including hotel and air fare.

Every day, Stand-Buys Ltd. offers its members cruises, charter air fares, and vacation packages at bargain rates. It serves as an information clearinghouse for tour operators who have not been able to fill all the airplane seats or hotel rooms they have reserved. Some operators cut their losses by selling these unfilled spaces at a discount at the last moment.

Members save an average of 35 percent off the regular price, the company says. Membership costs $45 a year per family.

The ''last minute'' vacation idea seems to be catching on.

Other companies are using similar approaches. And membership at Stand-Buys, the first and only nationwide clearinghouse, is growing, says Ann-Marie Taylor, general manager of the company, which is based in Southfield, Mich., and is a subsidiary of the Greatways Corporation.

''It kind of spread word-of-mouth in the travel industry,'' she says. ''We've experienced a lot of growth in just the last three years.'' Stand-Buys currently has a little over 20,000 members. It hopes to double that figure next year through an aggressive marketing program.

The company operates a hot line with updated recordings of some 20 discount trips available. Charter air fares are usually announced three days to three weeks ahead of time. The lead time for cruises is up to five weeks.

The 60 or so tour operators who work with Stand-Buys are not eager to publicize the service. But they are apparently satisfied with it. One large Midwestern tour operator has called Stand-Buys ''fantastic.''

Last month, Mr. and Mrs. Vernon Strobl took advantage for the first time of a Caribbean cruise offered by Stand-Buys. They had four weeks' notice.

''Everything went just beautifully,'' says Mrs. Strobl of Chelmsford, Mass. ''We didn't have too much time to think about it or worry about it.

''Most of my friends who have taken cruises have planned six months in advance.''