Since the minimum operating voltage for the LCD is (VDD - Vo), and VDD = 5V, VO must be less than 0.5V.
But the maximum operating voltage for the LCD could be 5.1 volts which would put VO at a negative value. Typically a negative value is only needed for the extended temperature LCDs but it is still a possibility for yours.
To check this out all you have to do is disconnect the GND end of the potentiometer and connect it to the negative terminal of a battery. Connect the positive terminal of the battery to GND and you can now vary VO from +5 to -whatever size battery you used.
Don