Hello,
Dont loose heart.
Those who keep Trying Never Loose!!
I am a R8C user too. We I have used R8C/13.
You can either use the internal ref or use the VCC for external ref. But mind it the Vref < VCC - 0.5 else I have faced a lot of problem in one of my projects. I was trying to make a scope to show the Voltage signal that I measure. I was having a lot of the offset problems. Then I simply attached a 220R resistor in series with the Vref. This worked. Probably you too can try it out.
Best of Luck!!