Half-baked but occasionally hilarious, "National Lampoon's Vacation" gets a reasonable amount of mileage out of depicting the disaster-prone journey of a "typical" family of four, the Griswolds of suburban Chicago.

After purchasing a new station wagon, they have two weeks in which to break it in on a "dream" trip to Walley World, an amusement park in Southern California. Naturally, the dream turns into a recurring, disillusioning nightmare of breakdowns, mishaps, humiliations and unforeseen expenses, but they contrive to outpluck adversity and gain a desperate measure of gratification upon reaching their destination.

John Hughes' screenplay is a nominally updated adaptation of his original National Lampoon story "Vacation '58," one of the best comic fantasies the magazine ever published. Reprinted in the July issue to help promote the movie, it still reads beautifully and enjoys a stylistic concentration that the movie, subject to the collaborative input of several humorists, fails to duplicate. There were two irresistible psychological keys to the story's effectiveness: The father of the family goes certifiably crazy as a consequence of the trip's frustrations and this process is described, with admiring affection, through the eyes of his adolescent son, who derives considerable pride and excitement from the sequence of events that drive dad around the bend.

It's roughly the first half of the movie that clicks. I think you can feel the continuity begin to fizzle during a protracted encounter between Chevy Chase, as the sorely tried Mr. Griswold, and Christie Brinkley, tacked onto the proceedings as a temptress in a red Ferrari. When she's finally obliged to do something more substantial than drive by with a flirtatious smirk on her face, it becomes tediously apparent that Brinkley is a decorative washout of a casting stunt. She doesn't have an essential, amusing role to play in the hero's fantasies, and as an object of voluptuous erotic interest she compares poorly with Beverly D'Angelo, who's cast as Mrs. Griswold, has some performing rapport with Chase and even gives the filmmakers more uninhibited, undraped sex appeal to exploit.

This is certainly Chase's most likable vehicle to date, and he endows Mr. Griswold with a sincere sort of goofiness.

What jolts the second half out of the doldrums is not fresh comic business but crowd-pleasing spectacle: a pair of vicarious, vertiginous roller-coaster rides at Walley World, a transparent substitute for Disneyland, the actual destination in "Vacation '58." Although director Harold Ramis can't be trusted to sustain the narrative flow for longer than five minutes at a stretch, comic highlights emerge with giddy frequency from the episodes describing the Griswolds' departure, early vicissitudes and excruciating stopover at a Kansas farm, where they visit some alarming rural relatives and learn that they'll be expected to drop off Aunt Edna (Imogene Coca, underutilized) and her obnoxious pooch in Phoenix.

Ramis, best known for his ingratiating performance as Bill Murray's pal in "Stripes," has directed only one previous feature, the Chevy Chase comedy "Caddyshack." He remains a rather tentative, no-frills sort of comedy director. The mobile, scenic aspects of the trip are pretty much wasted on him and cinematographer Victor Kemper, whose panoramic shots look consistently pedestrian and bleached-out. In fact, the most stylish pictorial motif in the movie appears during the opening credits, which are superimposed over enlargements of postcards, and a director with more confidence or visual flair might have tried to echo some of this poster art. In close quarters Ramis does have a delightful flair for sight gags, and there's a classic sequence predicated on the idea that all the Griswolds have nodded off while driving at night, including Mr. Griswold, who's doing the driving. As a matter of fact, Ramis seems to get his MOVIES HIGHWAY HILARITY TAKING A 'VACATION' WITH CHEVY CHASE By Gary Arnold

Half-baked but occasionally hilarious, "National Lampoon's Vacation" gets a reasonable amount of mileage out of depicting the disaster-prone journey of a "typical" family of four, the Griswolds of suburban Chicago.

After purchasing a new station wagon, they have two weeks in which to break it in on a "dream" trip to Walley World, an amusement park in Southern California. Naturally, the dream turns into a recurring, disillusioning nightmare of breakdowns, mishaps, humiliations and unforeseen expenses, but they contrive to outpluck adversity and gain a desperate measure of gratification upon reaching their destination.

John Hughes' screenplay is a nominally updated adaptation of his original National Lampoon story "Vacation '58," one of the best comic fantasies the magazine ever published. Reprinted in the July issue to help promote the movie, it still reads beautifully and enjoys a stylistic concentration that the movie, subject to the collaborative input of several humorists, fails to duplicate. There were two irresistible psychological keys to the story's effectiveness: The father of the family goes certifiably crazy as a consequence of the trip's frustrations and this process is described, with admiring affection, through the eyes of his adolescent son, who derives considerable pride and excitement from the sequence of events that drive dad around the bend.

It's roughly the first half of the movie that clicks. I think you can feel the continuity begin to fizzle during a protracted encounter between Chevy Chase, as the sorely tried Mr. Griswold, and Christie Brinkley, tacked onto the proceedings as a temptress in a red Ferrari. When she's finally obliged to do something more substantial than drive by with a flirtatious smirk on her face, it becomes tediously apparent that Brinkley is a decorative washout of a casting stunt. She doesn't have an essential, amusing role to play in the hero's fantasies, and as an object of voluptuous erotic interest she compares poorly with Beverly D'Angelo, who's cast as Mrs. Griswold, has some performing rapport with Chase and even gives the filmmakers more uninhibited, undraped sex appeal to exploit.

This is certainly Chase's most likable vehicle to date, and he endows Mr. Griswold with a sincere sort of goofiness.

What jolts the second half out of the doldrums is not fresh comic business but crowd-pleasing spectacle: a pair of vicarious, vertiginous roller-coaster rides at Walley World, a transparent substitute for Disneyland, the actual destination in "Vacation '58." Although director Harold Ramis can't be trusted to sustain the narrative flow for longer than five minutes at a stretch, comic highlights emerge with giddy frequency from the episodes describing the Griswolds' departure, early vicissitudes and excruciating stopover at a Kansas farm, where they visit some alarming rural relatives and learn that they'll be expected to drop off Aunt Edna (Imogene Coca, underutilized) and her obnoxious pooch in Phoenix.

Ramis, best known for his ingratiating performance as Bill Murray's pal in "Stripes," has directed only one previous feature, the Chevy Chase comedy "Caddyshack." He remains a rather tentative, no-frills sort of comedy director. The mobile, scenic aspects of the trip are pretty much wasted on him and cinematographer Victor Kemper, whose panoramic shots look consistently pedestrian and bleached-out. In fact, the most stylish pictorial motif in the movie appears during the opening credits, which are superimposed over enlargements of postcards, and a director with more confidence or visual flair might have tried to echo some of this poster art. In close quarters Ramis does have a delightful flair for sight gags, and there's a classic sequence predicated on the idea that all the Griswolds have nodded off while driving at night, including Mr. Griswold, who's doing the driving. As a matter of fact, Ramis seems to get his best ideas when something goes haywire in the front seat.

The continuity of the film might be tightened by more recognition of the peculiar, manic urgency and ill temper that long car trips and superhighways can induce in family men. Mr. Griswold plans to do a roundtrip of about 5,000 miles within two weeks, a self-imposed ordeal that virtually dares him to relax and enjoy the trip.

"Vacation" is likely to generate considerable appeal among juveniles and probably will be forgiven its diffuse, sputtering attributes more readily by kids than their parents. Pointlessly, the filmmakers make it harder on themselves and their most promising public by going to market with an R rating. The sole cause appears to be an excessive reliance on the sort of profanity that makes an R more or less automatic. With this element slightly toned down the film probably could have coasted in at PG and still sounded racy enough for all practical purposes. As it is, the juvenile patronage may be so heavy anyway the distinction between the two ratings will get even blurrier. NATIONAL LAMPOON'S VACATION

Directed by Harold Ramis; screenplay by John Hughes; director of photography, Victor J. Kemper, A.S.C.; production designer, Jack Collis; edited by Pem Herring; music by Ralph Burns; produced by Matty Simmons. Presented by Warner Bros. Rated R. THE CAST Clark Griswold . . . . Chevy Chase Ellen Griswold . . . . Beverly D'Angelo Aunt Edna . . . . Imogene Coca Cousin Eddie . . . . Randy Quaid