You may very well have protection even if you can't measure for antibodies!
What a vaccine does is cause antibody's to be produced (think of if as an all points bulletin with map handed out to police, where police are T-cells) sufficient to get rid of the vaccine. The vaccine, after all, is not welcome in the body - so your body gets rid of it. If you body has difficulty getting rid of it, more antibodies are produced (which also is inflammatory!). Eventually your response is sufficient and the "infection" or vaccine is removed. What's left behind are a few memory cells (i.e. the all points bulletin is tacked up in the office in case needed again) so T-cells can be called to action quickly should re-exposure occur.
Given that you take 4,000 IU's per day vitamin D, you may very well have high D sufficient for rapid response. The faster the response in clearing the virus (or vaccine), the fewer antibodies needed.
It seems contrary to the idea behind a vaccine creating antibodies (which they do) - but fewer is better if fewer work - less inflammation. Vitamin D activates T-cells fast (when there is enough D) based on the antibodies created by the B-cells in response to an antigen (virus or vaccine). If T-cells are not responding quickly (i.e. as in you have low D), the body keeps making more antibodies to get things going. This is no doubt why people with low D catching Covid have such high inflammation which can lead to hypoxia. They are making antibodies and no one is listening (i.e T-cells not getting activated) so more are made.
Ironically - once the infection is cleared, what's left are all those antibodies and T-cells which have to be told to stand down (i.e. go away) and vitamin D comes into play to activate other T-cells which do just that.
We're all different - some people will show higher antibodies than others in response to the vaccine - but I suspect people with adequate and high normal vitamin D would not have much antibodies in response - whether it be the flu, colds or other respiratory virus'.
I hope my explanation above is clear - I'm trying to avoid the medical jargon. Immunology is complex - but bit by bit, we're sorting it out (especially as it relates to cancer).