what adc are you using?

whith my experience on the ADC 0816 Vref will be for full scale count.
0V=00000000 while 2.5V=11111111

if you whant to measure higher voltages simply use a voltage devider on the analog in. The ratio will be the same. I have measured up to 299V with a Vref of 2.5V.

One option for Vref is to use a programmable regulator like the LM317.
or if i am not mistaken there is a Maxim chip that would give you a ref of 2.5V ( Forgot the name though check the website ) or simply use a 2.5V Zener.