If your parrot is healthy and on a good diet (particularly formulated/pelleted) this should not be a concern at all - they'll get sufficient vitamin D.
Do NOT give vitamin D made for humans to your bird unless it's under direct advice from a vet. I imagine your vet would provide you with another source if they felt it was warranted. While rare (in humans) vitamin D can be toxic if you get too much. How much is too much? That depends on your size, your metabolism, and many other things. Considering those tablets for humans contain 30-60% of YOUR recommended daily value of D think about what that would be for a bird. How many times heavier are you than your bird? 150 times, 300 times, more? One human vitamin D tablet could then be 18000% of their daily value... or 180 times as much as they should get. Note these numbers are just to make the point - their metabolism and physiology is so different that they may need more so it might not be this bad ... but maybe they need less and it could be worse!
Unless you can figure out how to divide those human tablets into 20,000 pieces I wouldn't give them any... even if I could do that I still wouldn't.
If you think your bird may not be getting enough vitamin D first check their diet, then ask your vet, then only if their diet seems lacking and the vet recommends it should they be given any. But still you'd have to find birdy doses, not human doses.
They do sell vitamin D supplements for birds in pet stores so that takes care of one of the issues - use that, not human supplements - but still only if needed.
I am not a veterinarian (though I've worked with several), but I think I can safely speak for nearly every vet on the planet in saying IF IT AIN'T BROKE, DON'T FIX IT.
So many health problems in pets are caused by owners trying to 'treat' their pets for problems they don't have.
Kaslo, you did mention your conures diet in another thread. Does she also get pellets, or is it all home made? If it's all home made that can be great, but then you do need to pay more attention to nutrient balance and it would be worth looking into how much vitamin D she is getting. The best natural sources of vitamin D for a bird may be various fish (salmon, mackerel, tuna). If you feed exclusively or primarily a home made mix you should talk to your vet about recommended values of various nutrients... don't expect them to know off the top of their head or have an easy answer - but do expect them to find a good answer for you.