Using the AD5620 or AD5640 with Vref=1.25v.
1- According to the datasheet Vref pin must be shorted with Vout and it gives a maximum output voltage of 2xVref with resolution of 2xVref/2^N.
Question: Is there a possibility of connecting Vfb to GND or unconnected or any other option for getting maximum output voltage of Vref with resolution of Vref/2^N?
You need to close the opamp loop so, the answer is no.
Say that, you can use the AD5683R....