What foods should I eat while taking radiation for vaginal cancer, also what are the best drinks to drink?

Depends on symptoms. Treatment with radiation towards the pelvis may have effects on the bowel. If diarrhea develops we suggest a low fiber diet. If a patient becomes fatigued we try decreasing carbohydrates and increasing protein. For most a balanced nutritious diet with variety works fine. All of this is tempered to not change a diet for a diabetic or other specialized diets when needed.
We're not sure. While we know that a person's diet does influence their overall health we do not really know whether certain foods are "better" than others during radiation treatment. In general, as the organs in the pelvis (including rectum and bowel) will be included in your treatment and this could result in some diarrhea we often suggest a low fiber diet so as to not worsen any possible diarrhea.