Positron-lifetime measurements in electron-irradiated pure Nb and Ta show that monovacancy migration occurs at 220 and 260 K, respectively. Hydrogen impurities can be bound to vacancies, as is experimentally observed in Ta at 70 K after low-temperature ±-particle irradiation. The vacancy-hydrogen complex formation shifts the vacancy migration to higher temperatures. Vacancy hydrogen complexes retain the capability to trap positrons. Theoretical calculations performed for hydrogen and positron states at vacancies are in agreement with experimental findings.