Which vitamins strengthen bones?

Many. The most important is of course vitamin d. You should have a vitamin d level of 50 for good bone health. 2000 iu daily could get you there if not overweight. Orthosilica, a mineral, is the rate limiting nutrient for bone collagen formation. It is the collagen that holds calcium in place in the bone matrix. Calcium is also important as are boron, magnesium and vitamin c.