There is no such thing as the "stomach flu" so of course the flu vaccine won't help. Flu is strictly upper respitory -- Not intestinal.
As with almost any infectious illness - Hand Washing Hand Washing Hand Washing. Here's what the CDC says:
Keeping hands clean is one of the most important steps we can take to avoid getting sick and spreading germs to others. Many diseases and conditions are spread by not washing hands with soap and clean, running water. If clean, running water is not accessible, as is common in many parts of the world, use soap and available water. If soap and water are unavailable, use an alcohol-based hand sanitizer that contains at least 60% alcohol to clean hands. [meaning -- Purell is NOT as effective as good old fashioned soap and water.]
When should you wash your hands?
*Before, during, and after preparing food
*Before eating food
*After using the toilet
*After changing diapers or cleaning up a child who has used the toilet
*Before and after caring for someone who is sick
*After blowing your nose, coughing, or sneezing
*After touching an animal or animal waste
*After touching garbage
*Before and after treating a cut or wound
What is the right way to wash your hands?
*Wet your hands with clean, running water (warm or cold) and apply soap.
*Rub your hands together to make a lather and scrub them well; be sure to scrub the backs of your hands, between your fingers, and under your nails.
*Continue rubbing your hands for at least 20 seconds. Need a timer? Hum the "Happy Birthday" song from beginning to end twice.
*Rinse your hands well under running water.
*Dry your hands using a clean towel or air dry them.