The trapping rate of positrons into small vacancy clusters and light substitutional impurities in metals is calculated. The host metal is modelled by a uniform jellium with a spherical cavity describing the vacancy of the vacancy cluster. In the case of a substitutional impurity a point charge is placed at the centre of the vacancy cavity. Self-consistent electron densities are solved in the local density approximation for exchange and correlation. The corresponding localised (trapped) and delocalized positron states are calculated using a local form for the electron-positron correlation. The trapping rate is calculated as a function of the initial positron energy and for thermalized positrons as a function of temperature. The model predicts that due to scattering resonances the thermal trapping rate for some small vacancy clusters may exceed that for a single vacancy by more than an order of magnitude. The effect of light impurities H and He is seen to be rather small. The calculations show that the trapping rate is a quantity sensitive to the details of the trapping potential and predict that the experimentally observed trapping rates and their temperature dependence can be very different in different materials.