Is this an ad hoc fallacy or a bunch of fallacies? Or is this a problem with using induction in the wrong context.

Person 1: Vaccines don't work and maybe make you sick. 

Person 2: Then why does the overwhelming scientific consensus think they do work and don't make us sick? 

Person 1: Well let me rephrase "maybe" they don't work. You can't tell the difference in those studies and Anti vax studies big pharma has a special interest.  I'm not asserting anything I'm just saying its possible the Vaccines don't work I'm not saying its certain. And theirs virtually no way of knowing.

If your wondering yes this was an argument I had with an antivaxxer word for word :(

Well let me rephrase "maybe" they don't work.

They didn't say maybe they don't work... they said maybe make you sick. So they are pulling a switcharoo.

You can't tell the difference in those studies and Anti vax studies

I don't know what this means. Of course you can tell the difference between working and not working in double-blind placebo controlled clinical trials—which what COVID vaccine research was. I know of no "anti-vax study" that did anything like clinical trials.

big pharma has a special interest

ad hominem (circumstantial). Special interest doesn't mean fraud. Family doctors have a 'special interest' in keeping you healthy—and they get paid for their services.

 I'm not asserting anything I'm just saying its possible the Vaccines don't work I'm not saying its certain.

And it is possible that the moon is made of green cheese. appeal to possibility 

And theirs (sic) virtually no way of knowing.

Yes, there are many ways of knowing with about as much confidence as the effectiveness of any successful drug science has produced.

It sounds like this person is on the last phase of denial before accepting reality, which is typically holding on to the concept of "it is possible that," despite all evidence the contrary, knowing that science doesn't deal in certainties.