To the Nike employees saying "vitamin D is totally safe and legal": consider the dose.
The amount you get in a glass of milk is 400 iu.
The amount of a typical pill taken daily is 1000 iu.
You can also buy non-prescription pills of 5000 iu.
Much research suggests that 3000-5000 iu/day is needed to maintain healthy levels. This can be received through sun, supplements, or a combination of both. Note that in northern latitudes (most of the US and Europe) you generate none from the sun from November to March. But your body will store surplus (gained in the summer) and access it through the winter.
A day in the sun during the summer will generate a max of about 20,000 iu.
For someone extremely deficient in vitamin D, a prescription dose of 10,000-40,000 iu/day is given for a few weeks to restore healthy levels.
A daily dose of 40,000 for more than 1-2 months is toxic for most people.
If Mo was taking a dose thought to be harmful to his health, that means it was a daily dose of 40,000 for many months, or a higher daily dose for a shorter time.
In other words, a dosage 10x higher than any normal person would take for health reasons.
Yes, there are reasons to be highly suspicious of why he was taking this. Likely because some other drug was causing an extreme depletion of his natural stores.