As Vref input of your ADC can look like a 1kOhm resistor, a current of up to 5mA will be drawn. So, MAX232 could eventually be used to deliver this current. But keep in mind, that then transciever output voltage swing might no longer fullfill RS232 standards! And, proper filtering is needed to suppress switching spikes!

Reference voltage could be fabricated by the help of TL431 (or similar), mounted as simple shunt regulator. 'Precision' should be high enough for your 8bit application, and an additional operational amplifier wouldn't be needed either. What is total current consumption of Vcc?

Another option is to use a reference voltage of less than 5V. Then again TL431 (or similar) could used to fabricate the reference voltage and drawing supply current from MAX232 would no longer be needed.

Kai