Not sure I understand... Have you connected Vdd to an analog pin or are you trying to somehow "internally" get a reading of it?

If you're using Vdd as Vref for the ADC then it simply won't work because whatever Vdd is is also your refenrece for full scale ADC reading. You'd need to configure the ADC to use external Vref and have a stable voltage reference that doesn't change as the battery voltage goes down (voltage reference IC, zener diode, voltage regulator etc). Then you can use a resistor network to "scale" the voltage to withing the Vref- and Vref+ for the ADC.

Does that make sense or did I completely miss the point?

/Henrik.