That's an interesting hypothesis. Certainly - as Bret Weinstein and others have said repeatedly - official ignorance of the clear protective role of vitamin D in covid infection is effectively criminal negligence.

It would be really easy to test whether vaccination and/or infection reduces vitamin D levels and for how long. Until we have such data I'm not inclined to suspect that vitamin D is a primary driving factor.

It's almost certainly not going to be a sequential step down like that, because serum D levels will tend to recover after each acute immune insult. If levels don't recover, there would need to be some other driving factor - e.g. persistent immune activation that is continuously consuming D - in which case vitamin D deficiency would be a the second-order effect.