I hope everyone agrees that Vitamin D is an important component of the diet. It helps regulate the transportation of calcium from soft tissues to hard tissues (ie bones), is a factor in immunity (not only having to do with fighting viruses and bacteria but also combating cancer). It is required in so many mechanisms that its deficit represents a serious health and cognitive risk.
Before addressing the validity of using people evolved to live in a particular country as the test subjects, let me first address why we can see varying levels of deficiencies around the world. Vitamin D is naturally produced by the body by exposure to the sun. On the planet, there is a wide variation of light that people would be subject to. Obviously light exposure in countries closer to the poles would receive less light compared to countries closer to the equator ... especially during winter months.
There are three things which are counter to the logical assumption that people in these areas would have variance in their Vitamin D levels. The first is an evolutionary response. People who evolved in lower light conditions have less skin pigment to affect the absorption of sunlight. Conversely, people living in high light conditions have more skin pigment.
The second is a behavioural response. Certain cultures cover a larger area of their skin with clothes. That is why in areas of the middle east, women have higher Vitamin D deficiencies. This would be especially true if they rarely go outside. This explains why there are so many naturists in northern countries. While most of the year they need to go clothed due to winter conditions, they will walk around naked when they have a chance during the summer.
A third condition that counters low Vitamin D levels have to do with the supplementation movements which exist in certain countries. Decades ago Canada recognized that everyone needs to have Vitamin D supplementation to prevent rickets in school children. Consequently, milk was selected as the food to receive the supplementation ... under the assumption that it was an inexpensive and easily accessible food for all people. This is true only to the extent that there is a certain mutation that allows certain humans to consume milk past a certain age. This mutation appears predominantly in people who migrated through the middle east about eight thousand years ago. Specifically, people who eventually came to live in northern Europe and their descendants carry this gene. This is why most people in Africa, Asia, Middle and South America do not carry this ability. This dairy supplementation continues to be effective for the infants and the children but is not for adults who don't drink milk due to lactose intolerance.
This draws us back to the criticism about using non-South Asians to test for the half-life of Vitamin D. Presumably a sample of British people with north European ancestry would be very similar: genetically, culturally and behaviorally. They would all have similar diets (with the exception of fibre), they would be fairly homogeneous in clothing style, they would all spend a similar amount of time in the sun. Being South Asian is much more genetically diverse. Evidence from mitochondrial DNA shows that India (South Asian) was populated by two different routes - a northern and southern migration. Not only are there differences in evolution (it has been over 40,000 years) as groups populated specific areas, but also a cultural constraint due to the caste system.
The variable that was being considered was between fibre in the diet vs half-life. One type of Vitamin D has a half-life of 15 days. So if I take 100 units today, in 15 days I will have 50 units and in 15 days I will have 25 units etc. High fibre diets reduce the absorption of fat-soluble vitamins. So instead of absorbing 100 units today, I would only have 50 units today, 25 units in 15 days and 12.5 units 15 days after that. Vitamin D is a cofactor in many mechanisms. High-fibre diets also affect magnesium ... another cofactor. It might mean an increase in Vitamin D is necessary to compensate for deficiencies in other cofactors.
"White" people need to consume 20 IU of Vitamin D per lb (0.45 kg) in Britain to supplement the light levels to achieve the optimum Vitamin D level. A 100 lb (45 kg) "white" person would need 2000 IU on a regular diet. On a high fibre diet, that person might need 4000 IU to have the same effect. People with darker skin would need 25 IU per pound of supplementation to achieve the same level... meaning a 100 lb person would need 5000 IU to have the same effect on a high fibre diet. A woman with darker skin but fully clothed with face coverings (as many South Asians are) would have no benefit from natural sunlight so she would need to have received all of their vitamin D from their diet and supplementation.
How this impacts Vegans/Vegetarians. Vitamin D3 is derived from animal-based products while Vitamin D2 is derived from vegetable-based sources. The liver metabolizes both forms of D but Vitamin D3 is twice as effective as D2. Vitamin D3 is created in your skin so your body is evolved to metabolize it better. Vitamin D2 is created in a similar process in plants and mushrooms. It is close to Vitamin D3 but insufficiently so to allow proper metabolism. Vitamin D2 degrades faster than Vitamin D3. So a 100 lb person with limited light exposure, on a high fibre vegan diet might need to take as much as 20,000 IU of Vitamin D2 per day in Britain in comparison to a caucasian with average light exposure, average fibre mixed diet receiving 2000 IU of Vitamin D3.
In general, people can do whatever they want to do. I really don't care if people choose to be vegan or immigrate to different countries. That being said, there is a difference between informed or uninformed decisions. If one chooses to be vegan, it is a decision made with the recognition that one must make the necessary dietary and supplementary choices to ensure one's own health and the health of others.