He didn't say he was forced to use the 5v as Vref? Just that he didn't want to use opamps?
It's pretty easy to use something like a 1.8v blue LED and a resistor to be a 1.8v Vref and use that for the ADC unit. The datasheets usually say 2v is the minimum that Vref should be used, but this is to ensure full linearity at full range, especially the top of ADC range. For an LM32 that puts out 0.020v to (say?) 0.800v a 1.8v Vref will work fine and be linear enough.