The story of star-crossed romance, ''A Month by the Lake'' focuses on four characters who almost - but not quite - pair off with the wrong partners. Vanessa Redgrave plays Miss Bentley, an Englishwoman who refuses to let her advancing years put the slightest damper on her enjoyment of life, love, and happiness of all kinds. A vacation on Italy's gorgeous Lake Como introduces her to Major Wilshaw, played by Edward Fox as a slightly befuddled gentleman who'd make an ideal mate for her. That is if he weren't so dazzled with the fluttery Miss Beaumont, a younger woman whose flirtatious ways give him the mistaken idea that she's set her cap for him. Rounding out the quartet is youthful Vittorio, a local resident who falls for Miss Bentley despite her age and her best efforts to put him firmly off her trail. It's fun to watch Redgrave tackle a role that's almost pure froth, and Fox gives the Major a blend of competence and clumsiness that's ripely amusing at times. They're well supported by Uma Thurman, a suitably fetching Miss Beaumont, and Alessandro Gassman, a winning actor (son of Vittorio Gassman, the great Italian star) who shows all Hugh Grant's charm minus the cloying mannerisms. The grand actress Alida Valli also makes a brief appearance as the hostess of the Lake Como resort. Aside from Pasqualino de Santis's attractive camera work, however, these performances are the picture's only real asset. Directed by John Irvin with a self-consciously playful touch, ''A Month by the Lake'' is clearly another entry in the ''Enchanted April'' sweepstakes. It hopes to cash in on the recent vogue for sunny entertainments that often care more about exquisite scenery than imaginative storytelling. The plot is so lightweight that it fades from memory while it's still going on. Despite its effort to evince a poignant nostalgia - with its idyllic setting in Europe just as World War II is about to erupt - there's precious little happening beneath its smoothly calculated surface. Squarely aimed at moviegoers who enjoyed ''A Room With a View'' and ''Four Weddings and a Funeral,'' it serves mainly as a reminder of how much fresher those trend-setting entertainments were.