We describe an approach to the positron trapping at vacancies based on the optical theorem. We calculate the imaginary part of the optical potential in an ab initio model at the time when electron - hole excitation takes place. The obtained analytical formulae for the optical potential are tested as a function of the positron incident energy, the positron binding energy, and the electron density parameter . The calculation of the optical potential range agrees well with the vacancy radius. The approach is used for the description of observed temperature dependences of trapping rate at vacancies in pure metals as reported in the literature. The results are satisfactory and the obtained values of the optical potential are quite reasonable.