Datasheet:

bit 6 VCFG: Voltage Reference bit
1 = VREF pin
0 = VDD


So you have to change your
Code:
ADCON0      = %10001101
to
Code:
ADCON0      = %11001101
AND connect Vref pin to your separate voltage source (since you want to use Vref pin, you should have a voltage source other than Vdd).