It sounds like you are using an XBee Zigbee (S2) rather than an XBee 802.15.4 (S1) or an XBee Digimesh 2.4 (S1).
The XBee Zigbee (S2) does not accept a VREF and it is recommended the pin just be connected to VCC. If you need VREF you would look to use a different radio model.
See http://ftp1.digi.com/support/documentation/90000976_U.pdf Page 14.