To obtain vacancy-solute binding energies in Al, we employed coincidence Doppler broadening (CDB) method of positron annihilation at high temperatures. By measuring the elemental specific momentum distribution of core electrons in high momentum region, we tried to observe directly vacancy-solute complexes in thermal equilibrium states. However, in the case of a dilute Al-Zn alloy, no vacancy-Zn complex was detected within an experimental accuracy, strongly suggesting that the binding energies are vanishingly small. The small binding energies are consistent with the previous thermal equilibrium experiments, but not with the quenching ones.