Will your bearded dragon die from not getting vitamin d3?

It's possible. Lack of vitamin d3 leads to Metabolic Bone Disease (MBD). If left untreated MBD can be fatal. The best way to avoid MBD - is to ensure your Beardie has the correct (natural) foods, supplemented with a broad vitamin powder. Additionally, you should have an approved UV light in the vivarium. The UV light synthesises vitamin D from the lizards diet.