Atomic defects can be generated relatively easily in phosphorene due to their low formation energies. How these defects affect the buckling behavior of phosphorene nanotubes (PNTs) remains unexplored. Using molecular dynamics simulations, we investigate the effect of vacancies on the buckling properties of PNTs. We show that compared to a pristine PNT, the defective one exhibits a much lower buckling strength and strain. Remarkably, 1% concentration of vacancies in a PNT is able to cause a 30% reduction in buckling strength. Interestingly, for long PNTs, the buckling occurs via column or global buckling. As a result, the buckling strength decreases significantly with the increase (decrease) in the tube length (diameter) for both the pristine and defective PNTs, consistent with the Euler buckling theory. For short PNTs with small slenderness ratio (L/D), however, buckling occurs via shell or local buckling. As a result, the buckling strength increases with decreasing the tube diameter, consistent with shell buckling theory. Finally, with the increase in temperature, the buckling strength and strain can be reduced significantly for both the pristine and defective PNTs. These findings may provide important guidelines for the design and applications of PNTs-based nanodevices.