June 24, 1981

VACATIONS often spell disaster even for the most disciplined of diners. Confronted three times a day with calorie-laden restaurant menus, many throw caution to the winds and come home five or more pounds heavier and sometimes with raised serum cholesterol and blood pressure to boot. Even those who try to make sensible choices are often foiled by the chef, who may add a tablespoon or two of butter to an otherwise leansounding fish or chicken dish, or spike the cold vegetable soup with a generous dollop of sour cream, or drown the low-calorie salad in an oily dressing. And it's usually impossible to tell from the menu - especially menus written in a foreign language - exactly how a dish is prepared. Breaded and fried or sauteed in butter or oil? In a fatty cheese or cream sauce? Seasoned with salty anchovies? Doused with a rich gravy? Topped with whipped cream? Even those who choose wisely often end up eating more than they really want because they suffer from the ''I'm-paying-for-it-anyway, I-might-as-well-eat-it'' syndrome.

-------------------------------------------------------------------- Robert Farrar Capon, an Episcopal priest and food writer, is the author of ''Supper of the Lamb'' (Harcourt Brace Jovanovich). By ROBERT FARRAR CAPON THERE are four things that summer's heat causes to flourish: a refrigerator with more food after dinner than before; cooked pasta in countless shapes but short supply; the scrap of steak that sits solitary in plastic wrap; and company that arrives unannounced. Put them all together and they spell leftover cookery: to some a curse, but to those with the wisdom to resist vain repetition a sovereign deliverance from the bondage of recipes into the freedom of invention. God does nothing twice: He makes no two snowflakes alike and never sends the same high tide again. And man, in His image, goes ill unless he goes with the flow of change. Only fools try to repeat a party; any gravy that is not doctored the second time around becomes inexorably worse.

In the Broadway column in the Weekend section last Friday, the lyricist for ''Jean,'' a new musical that Michael Bennett may direct next spring, was reported incorrectly. The lyricist is Christopher Adler.

Cyrus R. Vance, the former Secretary of State, has sharply criticized the Reagan Administration's decision to sell arms to China as ''needlessly provocative.'' Mr. Vance, who was in Moscow just before the new policy was announced, said that the proposed arms deal has ''virtually removed any influence we have left over the Soviet Union.'' ''We played the China card in no-trump,'' he said in an interview Monday, ''and there is not much left.''

SALMON was once a cheap fish, as prevalent as chicken today. It was so plentiful that servants in Colonial America had clauses in their contracts stipulating that they could not be served salmon more than a certain number of times a week. Two centuries before, Elizabethan servants had objected to too many oysters. No wonder you can't get help these days. Only in the last 25 years, because of dwindling numbers, has salmon climbed the culinary scale to become a luxury. At elegant spring and summer dinner parties it appears garnished with truffles and lobster sauce or in brilliant aspic jellies on the buffet table. In New England poached salmon, fresh green peas and boiled new potatoes are the traditional Fourth of July treat. Salmon steaks are served with hollandaise sauce or cold with cucumber salad. One of the most popular dishes on the menus of New York restaurants this summer is raw salmon, cut paper thin and served with a mustard dill sauce - or even plain, nouvelle-cuisine style, with nothing but lemon and freshly ground pepper.