Addition of impurities to vanadium can substantially enhance its swelling. However, the swelling is accelerated only by `undersized' and not by `oversized' impurities. In this report we propose a mechanism of swelling acceleration in vanadium, which is based on the impurity effect on the elastic interaction of dislocations with point defects. It is shown that the segregation of impurity atoms at dislocations modifies the dislocation bias, the sign of the bias correction being opposite to that of the impurity misfit. Such bias modification should affect the swelling in qualitative agreement with the existing experimental observations for vanadium alloys.