i dont think there is a hard rule on vref+ and vref-
it would also depend on what order your ADC is. For a simple 1 bit ADC,
general rule of thumb is about 70% and 30% of your supply voltage.
(assuming 0-supply voltage is your input range)
I dont know if this is a good explanation, but let me give it a try.
you want to integrate the difference between input and vref, so you want (input - vref) to be as big as it can be to help the integrator reach 0.
For that, its better to choose vref+ to be between 50% to 100% of your supply
and vref- to be between 0 to 50%.
well, i would think the best results are achieved at 75% and 25%, but during my simulation long ago, i found best results at 70% and 30%.
I hope this helps.