Testing is the key here.
The approved covid vaccines had to go through rigorous controlled testing before they were approved.
And the up front testing and data still being gathered as millions are being vaccinated every day confirms that the approved covid vaccines are safe and effective.
According to the NIH links it looks like testing is currently under way for both Ivermectin and Vitamin D.
Ivermectin looks to me like hydroxychloroquine part 2, but we need to wait for rigorous testing to tell the story.
Like I said, based on what little I know at the moment, I would choose Vitamin D over Ivermectin (once again "ask your doctor first"). There is conflicting data out there on the effectiveness of Vitamin D against Covid-19. Which is why it needs to go through rigorous controlled testing. But if a person does not have access (or does not have access yet) to an approved covid vaccine, Vitamin D might be a temporary solution until vaccines that have been proven to be safe and effective are available (again "ask your doctor"). And if rigorous controlled testing demonstrates that Vitamin D is actually beneficial in any way against Covid-19 then that will be good news.