My opinion on EN is that they have great, reasonably priced cages, and interesting, if overpriced toys. As far as treats and food, I wasted a lot of money on things my gliders refused to eat.
I would never give my babies extra vitamins, it's all in their Wombaroo powder. Maybe it depends on the diet, mines eat OHPW/Critterlove.
If a vet suggested extra vitamins, do your research first. How much glider experience does the vet have?