We experimentally investigate single-drop formation from a vibrating capillary tube immersed in a second immiscible liquid. At set forcing frequencies, significantly smaller drops are generated when a threshold amplitude is reached. We show that a drop grows and resonates in first mode once the drop resonance frequency and the forcing frequency coincide. The drop then detaches by a stretching mode if its elongation ratio exceeds a critical value, function of the drop to pore diameter ratio. The detached drop diameter is well described by the Bisch et al. expression for bound drop resonance frequency. Otherwise, the drop grows, leaves the first-mode resonance range and detaches at a larger size by dripping. The threshold and drop diameters are well predicted by a simple LFHO-based model.