My initial thought on reading your post was that there are probably more vaccine refusers than there are ivermectin believers (my initial thought was based on my conversations with people where either of the two topics have come up, which is admittedly a tine sample).
After some quick google research, I'm not so sure my initial thought was correct. One poll showed that of adults who've heard of ivermectin 33% believed it was either very effective or possible effective:
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Most Republicans who have heard a lot or a little about ivermectin say it is effective (56%), compared with 19% of Democrats.
today.yougov.com
Disclaimer: I know nothing about this website or it's polling methodology, it might be cr*p.
The CDC estimates that 87.6% of adults in the U.S. have gotten at least one dose of the vaccine.
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CDC’s home for COVID-19 data. Visualizations, graphs, and data in one easy-to-use website.
covid.cdc.gov
It's possible that the two circles in your proposed Venn diagram are almost the same size if the 13% "very effective" in the poll is representative of the general population and if this is equivalent to your proposed "people who insist Ivermectin is an effective COVID anti-viral" Venn circle (the two terms seem almost identical to me), and if the 12.7% (estimated) of the adult population that have not received one dose is accurate and if it's equivalent to your proposed "those who refuse to get vaccinated" circle.
Where am I going with this? No place really, but it does seem like a remarkable coincidence that the two groups appear to be almost the same size.
I suspect you're correct that there is a lot of overlap between the circles, but I don't know of a good way to prove or disprove it.