For the same structure, the absolute energy obtained by "qe" and "vasp" differs greatly, it seems that they have different definitions of energy, does anyone know the difference?
Absolute energy per se has no physical meaning if not as a convergence test. Energy differences are however important. You should check if QE and VASP give you the same energy differences, for example as the gap between occupied and empty states.
On the other hand, absolute energies might depend upon many parameters, for example, the pseudo potentials.