It just hinders the production of melanin

It is true that by blocking ultraviolet rays, sunscreen limits the vitamin D produced by the skin. Earlier it was presumed that the effect is significant. But more recent studies suggest that the effect is negligible. While sunscreen does hamper vitamin D production, these studies say, it is not enough to cause a deficiency. This in part may be due to the fact that most of the users typically do not apply enough sunscreen to get its full effects, which in turn allows some sunlight through, and on the other hand it may be so that it does not take much sunlight to produce adequate amounts of vitamin D: perhaps as little as 30 minutes of daytime exposure twice a week without sunscreen. But whatever it may be, it is better for those concerned about vitamin D to consume more foods rich in vitamin D, like salmon, milk and orange juice instead cutting back on sunscreen. Thus it may easily be concluded that sunscreen can reduce vitamin D production, but probably not enough to have a significant effect.
Requirements of vitamin D is according to the age of the individual.
Recommendations are as follows:
For Children: The National Academy of Sciences and American Academy of Pediatrics have recommended that all infants, including those who are exclusively breastfed, have a minimum intake of 200 International Units (IU) of vitamin D per day beginning during the first 2 months of life. In addition, it is recommended that an intake of 200 IU of vitamin D per day be continued throughout childhood and adolescence, because adequate sunlight exposure is not easily determined for a given individual.
For Adults: The Food and Nutrition Board of the Institute of Medicine has recommended the following as an adequate vitamin D intake: 200 IU daily for people 19-50 years old; 400 International Units (IU) daily for those 51-70 years old; and 600 IU daily for those 71 years and older.

The best way to get it is to take foods rich in vitamin D like milk or supplemented by cod liver oil , fish liver oil or one should take multivitamin tablets orally .

I guess not! Sunscreen is just a protection from UVA and UVR. Vit D is synthesize by sunlight but it doesn't mean that if you're wearing sunscreen the process of synthesizing vit. D will be stop.

Sunblock does prevent you from being able to convert vitamin D to the active metabolite that you need. But the good news is that you can get vitamin D from a lot of other sources. You can get it from foods; you can get it from a multivitamin.

Yes.
400 to 1,000 IU for adults under age 50.
Take in 1,000 IU — 25 micrograms — of vitamin D every day.

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“Osteoporosis Canada recommends daily supplements of:
• 400 to 1,000 IU for adults under age 50 without osteoporosis or conditions affecting vitamin D absorption.
• 800 to 2,000 IU for adults over 50.
The Canadian Cancer Society recommends Canadians take in 1,000 IU — 25 micrograms — of vitamin D every day. During the spring and summer, that can be accomplished through normal daily exposure to the sun. In the fall and winter months, a vitamin D supplement may be necessary.
Health Canada's guidelines remain at 400 IU — 10 micrograms. Health Canada advises people to take a daily vitamin D supplement, as following the revised Canada Food Guide would only provide 200 IU of vitamin D a day.”
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Read more: http://www.cbc.ca/health/story/2008/05/16/f-health-vitamin-d.html#ixzz0xGXA0mx8