blueroom: you snuck up from underneath me! Damn this slow BAUD RATE (300) forum
Yes you may. Vref- does not imply a compulsory negative voltage. This goes for electronics in general.
In section 15 of datasheet, get familar with the Absolute Maximum Electrical ratings.
The overall foundation to remember:
"Voltage on any pin with respect to VSS (except VDD, MCLR. and RA4) ....................................... -0.3 V to (VDD + 0.3 V)"
Then go to table 15-12 and read the valid ranged for Vref+ and Vref-. Vref-(min) cannot be less than Vss - 0.3v (within the overall foundation), in short not less than 0.3v. Vref-(max) cannot be larger than (Vref+) - (2.0v), 2 volts less then Vref+.