I have kept veileds for many years....and not had a case of MBD for a long time. This is what I do...
I use Repti-sun 5.0's. (My chameleons rarely go outside.) Since they only get artificial UVB, I dust their insects with a phos.-free calcium/D3 powder lightly twice a month. D3 from supplements can build up in the system, so don't overdo it.
I also dust twice a month with a vitamin powder that has a beta carotene source of vitamin A because it won't build up in the system. Its controversial as to whether chameleons can convert beta carotene, so some people use preformed vitamin A once in a while. Preformed vit. A can build up in the system, so don't overdo it. Excess vitamin A can prevent the D3 from doing its job, and lead to MBD.
I also dust at most feedings with a phos.-free calcium powder to help make up for the poor ratio of calcium to phos. that is found in many insects.
Gutloading is another important thing to do....be sure to take into consideration how much D3, preformed vitamin A, calcium and phos. there is in the things that you include in your gutload when trying to balance them.
If your chameleon has begun to develop MBD, which a vet should be able to tell you for sure, the vet can give her injections of calcium and, if need be, when her blood calcium levels are high enough, a shot of calcitonin to bring the calcium back into the bones.