Perhaps Vitamin D status is just an indicator of adequate sunshine on the skin. This would explain why people with low vitamin D are more susceptible, yet supplementation with Vitamin D is ineffective.
1. Vitamin D as a side effect of sun exposure is not itself the important indicator. Instead, some other biological response increases immune response simultaneously. Hence, D supplementation is itself copying the wrong response.
2. Vitamin D deficiency is caused by illness, hence it actually doesn't affect mortality and the relationship is not casual.
Don't screw around with sun protection. Despite what the internet says, people who work outside get it too.