Marry Christmas and Happy New Year everyone!

I am using pic12F675, picbasic pro compiler.
I use ADCON0.6=1 uses GPIO.1 as Vref then ADCIN 0, VA it works ok
then I use ADCON0.6=0 to use the Vdd as Vref then ADCIN 3, VA1 it will not work...no readings
Adin 0 uses external Vref and Adin 3 uses the VDD
how can I read both 0 and 3 with different Vref
I use the following:

ADCON0.6=1
ADCIN 0, VA
ADCON0.6=0
ADCIN 3,VA1

what is the problem? can't I use different Vref in the same program?!
Please help!