what if they're actually much healthier than average (because of other personal choices)? and what about all the other bad decisions people do wrt. their health (which is what causes the vast majority of healthcare expenditures)?

Those other situations should also be factored into the price they pay for healthcare. That being said, many of those other issues (addiction, obesity, etc.) are much more difficult to "prevent" than getting a vaccine. If there was a vaccination for those issues, then I would put them in exactly the same class.