It is known that the human body can produce vitamin D itself when exposed to enough sunlight. But will the body downregulate this natural production of vitamin D, if I take a vitamin D supplement? And if so, will the body increase natural vitamin D production again, when I stop taking vitamin D? Or will taking vitamin D permanently harm my bodies ability to produce vitamin D itself?
The body stops producing its own vitamin D when it has enough, whether it gets enough from supplementation/diet or sun exposure/natural means or a combination.
I'm on a nutritional therapy course now, and I've never read anything that says that is necessarily a bad thing (i.e., never read anything to suggest that supplementation will permanently shut down the body's ability to produce vitamin D for itself). That said, excess supplementation can cause problems with kidney function ("Excess vitamin D can cause the body to absorb too much calcium and can lead to kidney damage" - see link below), so it's not advised that supplements become the primary source of vitamin D.
I am not a doctor, but given that vitamin D synthesis is primarily driven by the activity and availability of specific liver enzymes, I highly doubt that it would be possible to down-regulate its manufacture in the human body through supplementation. If that were the case, then one would expect to see similar phenomena occurring with other endogenously produced substances when they are supplemented, and I've never run across a study demonstrating that this happens. Only a drug capable of inhibiting production of said liver enzymes could impact endogenous vitamin D production, and even this effect would be normally reversible upon discontinuation of the drug. Furthermore, vitamin D toxicity requires dosages that are orders of magnitude above the RDA, so if you limit yourself to reasonable quantities, I wouldn't expect any problems to crop up.