So it hinges on how different the psychological difficulties of accepting vaccination vs. "just eating less for years" are. Given how many vaccinations people already accept vs. other diseases, and the fact that any such irrational fears need to be surmounted just twice, as opposed to resisting (irrational) urges to eat every waking hour for years, it strikes me as being unlikely that even a serious fear of needles is even remotely as high a hurdle.
The thing is: people that are overweight but try to lose weight will nevertheless struggle. But certainly from how they talk people resisting vaccination are not trying to overcome their own limitations; i.e. the problems are not really comparable.
While there's a hypothetical world imaginable where the two would be equally difficult, it seems hell of a lot more plausible that the issue isn't a psychological hangup, it's conscious intent.
But if this is about people choosing to impose costs on others, then it's totally reasonable to force the issue or otherwise ensure that those making those choices bear the burden of the consequences - rather than innocent bystanders.
Insofar as people have real hangups, rather than making poor (but conscious) choices - sure, it sounds reasonable to help them - and that actually happens! People with fears of needles and the like can get extra help to get through the difficult (for them) ordeal, and perfectly fair.