Using empirical-potential and tight-binding models, we study the structure and stability of atomic-scale irradiation-induced defects on walls of carbon nanotubes. Since atomic vacancies are the most prolific but metastable defects which appear under low-dose, low-temperature ion irradiation, we model the temporal evolution of single vacancies and vacancy-related defects (which isolated vacancies can turn into) and calculate their lifetimes at various temperatures. We further simulate scanning-tunneling microscopy (STM) images of irradiated nanotubes with the defects, employing for this the tight-binding Green's function technique. Our simulations demonstrate that the defects live long enough at low temperatures to be detected by STM and that different defects manifest themselves in STM images in different ways, all of which makes it possible to detect und distinguish the defects experimentally.