My daughter was diagnosed with positive vasodepressor syncope and recently had an episode while flying. What can be done to prevent this?

Difficult. It can be very difficult to prevent. There's no easy answer or "cure." drinking plenty of fluids, eating more salt, & wearing compression or support stockings can help. Some medicines are helpful. Immediately lying down at first onset of symptoms will abort passing out. Avoidance of known triggers can help. She should see an electrophysiologist for refractory symptoms.