We present the energetic, structural and electronic properties that explain the accumulation of He inside a single vacancy in both a BCC W and a BCC Nb crystals. Using density functional theory, we have obtained the most stable structures for an increasing number of He atoms within the monovacancy, with and without the presence of van der Waals (vdW) interactions. Our results show that the maximum number of He atoms that can be placed in the monovacancy is nine and suggest that the vdW interactions should be taken into account for higher He concentrations produced in bigger n-vacancies. The analysis of the density of states and the He-metal interaction reveals a reduction of the repulsion as the origin of He trapping inside the metallic vacancy. The formation energy grows almost linearly with the number of He atoms included, showing a strong dependence on the bulk modulus, while the binding energy presents a more complex behaviour. Finally, the deformations of the first (1NN), second (2NN) and third (3NN) nearest neighbour atomic distances to the vacancy change in different ways. The maximum deformation obtained (around 0.5 ) is found with precisely nine He atoms inside the vacancy, just before the bubble collapses.