I never heard of a "bad lot" of vitamin D-3. It is much more effective than any flu shots, like 100%, at least among the couple dozen people I have got started on it. This year I gave bottles of it to some of the widows at church. I decided to do that when I heard an 80 year old lady complaining about the shot making her really sore and then getting flu anyway.
While I strongly support maintaining a good serum level of Vitamin D through supplements, because it has several actions against viruses, I would still recommend playing it safe by getting a flu shot.
Importantly, this year, the flu season in the southern hemisphere was correctly predicted, and of the (typically) three strains in the vaccine, one was accurate for the dominant type of flu, but a lot of people still got sick after vaccination.
However, flu is very, very tricky, in that it has a unusually large number of “flexible” genes, allowing it to mutate at great speed, to either a weaker or stronger strain. This means that vaccines are dependent on what I would call the “cousins approach”.
Say a vaccine is prepared for a given strain. It will likely also work well on mutated strains descended from it, as well as “first cousin” strains, for which it will provide partial protection. But when you get to second and third cousin strains, its protection becomes marginal or ineffective at best.
And the immune system works much the same way. The closer a strain is to one the immune system has previously experienced, the faster and more measured the response. A novel strain, however, might be all over the body before the immune system even notices it.
And *then*, the immune system might overreact, with what is called the “cytokine storm”, which can be more deadly than the flu itself. This happened most recently in Ukraine, and doctors were horrified that it made the lungs of the deceased look “burned”.
Vitamin D helps a lot, because it opens “immune pathways” to fight infections, a breakdown product of it erodes the viral coat in the bloodstream, and it also acts as an ACE inhibitor, which moderates the immune system and prevents it from overreacting.
However, it can only do so much. So think of it together with vaccine as a “shield and sword”. Vitamin D offers some protection like a shield, but you need the sword of the vaccine stimulating your immune response to that particular virus, to have a much better defense.