Yes. VREF is required during self refresh. All DDR components' on-chip address counters are still operational during self refresh mode, so VDD must be maintained within the stated data sheet limits.
Again, VREF must not be disabled after the DDR memory is put into self refresh mode. Doing so could easily result in inadvertently exiting self refresh.
You should understand that VREF draws almost no power; any current drawn by VREF is negligible when compared to VTT and the core VDD. DDR components typically use a differential pair common source amplifier as their SSTL_2 input receiver. Because the VREF pin is used primarily as an input to this circuit, its current draw is low. It is so low, in fact, that the device’s input leakage current (~5µA) can be considered the maximum current requirement for the VREF pin. Typical VTT power is drawn from other places on the board and depends on the other components used on the module/system in addition to DRAM devices.