I agree that most breeders do not sell baby buns until at least 4-6 weeks of age. But if they are eating well, I would not add in formula feeding or extra vitamins which are not very close to rabbit milk and could stress their GI tracts. I would recommend offering a good quality growth pelleted diet (such as Oxbow) to help balance their diet of hay. I hope that helps.
Pebble Creek Animal Hospital