No, you do not need to use all VREF's in one bank as VREF's. Each pin's reference to VREF is contained in a group. Therefore, if a certain group of pins are using a VREF, the other VREF's in the same bank may be used as regular I/O if their groups do not need them to be VREF's. You can find out the groupings of VREF's by looking at the second column of the Cyclone® series device pinout. It illustrates which pins use which VREF's.