We study vaccination as a public-good problem with epidemiological externalities and multidimensional private heterogeneity. A unit-mass continuum of individuals chooses whether to vaccinate. Vaccination entails a privately observed cost; remaining unvaccinated exposes the individual to an infection loss that depends on the aggregate non-vaccination rate via a simple incidence map. Motivated by phenomena highlighted during the COVID-19 pandemic, in particular the presence of medically vulnerable individuals who cannot safely risk either infection or vaccination and therefore require substantially enhanced indirect protection, we develop a framework that isolates and quantifies the size of a strategically vulnerable subgroup.