NXP PCA82C250 CAN controller interface
According to Fig 9 of that datasheet, it is for connecting to CAN controllers like P8xC592/P8xCE598 which have a CRX1 pin, which is for a differential receiver in the CAN controller, to compare against the RXD pin which connects to CRX0. So it is provided for convenience so that you don't have to implement a buffered half rail level to this sort of CAN controller. If your CAN controller sees RXD as a logic level using a single pin input, you don't need to connect Vref to anything.