Hi,
I've never used that chip but as far as I can see the fixed voltage reference can only be used as "input" to the comparator(s) in the chip - it can not be used as a reference for the ADC. The fixed voltage reference can however be measured by the ADC by selecting the proper "channel" via ADCON0.

Section 9.1.3 spells it out:
The VCFG bit of the ADCON0 register provides control of the positive voltage reference. The positive voltage reference can be either VDD or an external voltage source.
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Since the ADC is ratiometric but the 0.6V fixed voltage reference is not (I'm not sure about the 1.2V one) you should be able to compensate for any drift in Vdd (if that what you're using for VRef+) by sampling the fixed voltage reference and calculate the actual Vref voltage.

/Henrik.