FASTWIND predictions of the behavior of the lines involved in the Vz phenomenon (He i λ4471, He ii λ4541 and He ii λ4686) as a function of the wind-strength Q-parameter for different (T_eff, log g) pairs. A fixed projected rotational velocity (v sin i = 100 km s^-1) and resolution (R = 8000) have been considered. The range of log Q values for which a star would be classifed as a Vz star are indicated with the gray rectangle. The corresponding upper log Q value leading to the Vz characteristic is also quoted.