By the way, there is no such thing as a stomach flu. What you are describing is probably a mild case of food poisoning. It is very common. Has that mayonnaise gone bad? Check especially your meat, dairy, eggs, leftover rice (toss cooked rice after three days; sooner, if it's been left out), and things prepared with these ingredients. They're the worst offenders.
If your daughter's pee is not yellow, she is well hydrated. (Post-vitamin yellowing doesn't count in the hour or two after taking it.) If it is yellow at all, get her to drink more. The yellower it is, the more dehydrated she is. But be aware that there is such a thing as water toxicity, and it is fatal. Don't ever force (or bribe, or otherwise encourage) her to drink lots of water in short periods of time.
Don't force her to stay on the toilet. If you sense that she is holding it so that she doesn't miss any fun activities, you will need to work with her on this, but not by forcing her to stay on the toilet for extended periods. Get a discipline book like Easy to Love, Difficult to Discipline by Becky Bailey, or the Sears' Discipline Book for ideas that work and understand a child's thought processes around these issues. Also, you might be focusing on the wrong end! If she truly isn't going often enough, it could very well be a hydration issue. IF so, don't resolve it with high-sugar drinks (even 100% fruit juices are very high sugar; that is why fruit is sweet). Water them down (a lot - gradually increasing the water percentage, if you have to, if she's already used to regular juice). Sugar is extremely dehydrating.
L.