The uvb light should run at least 2/3 of the length of the tank. This ensures sufficient exposure to UVB rays your bearded dragon needs for proper health. As for substituting supplication of vitamin D3 for UVB light, I deffinitely don't recommend it. It is too easy to over dose your dragon with suppliments as it still isn't understood just how much vitamine d3 they need on a daily basis for proper metabalism of calcium. As most UVB bulbs do not produce sufficient UVB rays, we need to add calcium with vitamin d3 in our dragon's diet to ensure they are getting enough, especially with young growing dragons or gravid females.
About the only way to completely stop using UVB bulbs and supplimentation is to house your dragon out doors where they can access the sun directly and bask as much as they need to ensure their own health. Even then a multivitamin should still be offered regularly to ensure they have all the essential nutrients they need.
A healthy diet with as much variety as possible (both in insects (crickets, silkworms, superworms, butterworms, roaches etc) and greens/vegetables (collard greens, mustard greens, dandelion greens, green peppers, etc) ensures your dragon is getting the best you can offer in nutrition.
Unfortunately there is no 'cheap' way to ensure your reptiles health, when it requires uvb rays, you have to buy the expensive lights and replace them every 6-9 months. However, the cost of vet bills far exceeds the cost of proper food and lighting.
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PHLdyPayne