It Only Takes an Instant for Love to Strike   Tragedy hits the Allenhouse family on a hot summer night in Ohio when a mother of four vanished. Eight-year-old Vada virtually grew up overnight and raised her three younger sisters while her father lost himself in his medical practice in the basement of their home.   Now, Vada is a grown woman, still making her home with her father and sisters. Her days are spent serving as an errand girl for Cleveland's fledgling amateur orchestra; her evenings with Garrison Walker, her devoted, if passionless, beau.   Dizzying change occurs the day the Brooklyn Bridegrooms come to town to play the Cleveland Spiders and a line drive wallops the head of a spectator. The fan is whisked to the Allenhouse parlor, and questions swirl about the anonymous, unconscious man.   Suddenly, the subdued house is filled with visitors, from a flirtatious, would-be sports writer to the Bridegrooms' handsome star hitter to the guilt-ridden ballplayer who should have caught the stray shot. The medical case brings Dr. Allenhouse a frustration and helplessness he hasn't felt since his wife's disappearance. Vada's sisters are giddy at the bevy of possible suitors. And Vada's life is awakened amid the super-charged atmosphere of romantic opportunity.