olddudders
We are being told that Vit D is good for fending off the virus if we catch it, reducing the severity of the illness. High levels of alcohol reduce our ability to absorb Vit D - fact. So there is a non-tenuous connection, but a short abstinence is unlikely to make any difference if you are a heavy drinker, and none whatever if you are not.
I think you are indeed confusing Vitamin D and Vitamin B (various). Vitamin B is water soluble, so is constantly being excreted and needs topping up, whereas Vitamin D is fat soluble and the body builds up reserves.
Alcohol is known to deplete Vitamin B and heavy drinkers often have a deficiency. Not only does the deficiency have various symptoms, but it is used by the body when there's an infection.
It makes sense for the body to be in the best condition possible to cope with any change to the metabolism which might be introduced by a vaccine.
Abstaining for a couple of days might make a positive difference. It's certainly not going to do any harm - and will save a couple of pounds too!