I have an 8RDA+ rev 2.0 and setting vdd works well, but the scale seem to be off somehow.
What happens is
Setting -> Real Voltage reported from sensor
1.6v -> 1.6v
1.8v -> 1.616v
2.0v -> 1.808v
Maybe this stems from the undervolting the mobo already does by default, I dunno.
One more thing I've noticed is that when I set vdd the value in "Current
vAGP" is set to the same as "Current Vdd". This looks like a bug because if I reselect the AGP bar and press apply it is reset to the correct value again.