According to the datasheet, VREFA+ must be at least 2.4 volts, and can be up to VDDA. This is easy to ensure - you just link VDDA with VREFA+ even if you are not using an external precision reference.
Also, the datasheet explicitly states that VDD, VDDA and VDDC *MUST* come up in order for the device to boot at all (makes sense).
What is not clear is whether the device can boot with VREFA+ at 0V but with the rest of the part powered ?The datasheet doesn't suggest the part wouldn't boot - just that it would be outside of spec. The question being : what is the consequence of being out of spec ? If it will draw excessive current that would be bad. If it just didn't return sensible ADC readings, that would be fine.
Any insights appreciated,
Pat.