Each is a piece of the puzzle and has some effect. Vitamin D deficiency will lead to about a 12 times mortality rate and also likely more severe cases on average.... but I would not say it is based on immunity (for which you have none).
There are many pieces to the puzzle...
- UV radiation (sunlight) and ventilation significantly reduces the half-life of the virus on surfaces - which means that lowers the chances of catching it (lowering the R0 value - how many you infect on average if you catch it). The ventilation was found to have a very beneficial effect in regards to the Spanish Flu (something that is lost on modern hospital buildings).
- For your average Thai (outside of a few major centers like bangkok) they are more likely to live without constant AC and rely on ventilation (unless it gets really hot out) and will live a more outdoors lifestyle (especially when the malls and theatres are closed).
- Social distancing will reduce the opportunity for the virus to spread
- Wearing Masks will do the same as social distancing to an effect (masks reduce you spreading virus - about a 66% reduction I believe on ability to spread in lab tests; reduces it by about 33% for you catching it; and if both - it combines the two).
- Tracking and tracing contacts reduces the ability of it to spread when you are able track down someone infectious but not symptomatic (also reduces R0).
The problem is that in many western countries it got out of control and there is resistance to do more than light social distancing. That is the difference between South Korea and many western countries - they have all their eggs in social distancing which helps but without all of the mitigation efforts working together - you might have some success in flattening it but not really getting it under control and if they do it will take an extremely long time.
Deficient vitamin D levels (and some extent other nutrition) will compromise your immune system, but we will all start with no immunity - it is all about how quickly and how efficiently your immune system works once infected (with a few caveats)
BTW, The Thais do have significant co-morbidity issues do to their diet - higher salinity of the food which etc. ... There is a large under diagnosed or untreated issue with both hypertension and diabetes in Thailand (along with TB).