Is a VCCO supply required in an unused bank?

Yes, for best design practices, a VCCO supply is still required in both cases.

1. In the case of non-VCCO-dependent I/O standards, the input clamp diodes become forward-biased if the
VIN exceeds VCCO by the diode threshold voltage (~0.5V). This results in leaving VCCO disconnected or
setting it to an excessively low voltage that can lead to a distorted input signal.
To avoid this, VCCO should be greater than the input high voltage (plus any overshoot),
less the 0.5V diode voltage. For example, if the maximum input high voltage is 2.5V (including overshoot),
VCCO should be greater than 2.5V - 0.5V = 2.0V.

NOTE: The clamp diodes in Virtex-II /Virtex-II Pro/Virtex-4/Virtex-5 are not I/O standard-dependent -- they
are always present.

Additionally, a VCCO supply is always required to achieve proper termination on DCI inputs. For more
information, see the corresponded Virtex FPGA User Guide -> "Design Considerations" -> "Using
Single-Ended SelectIO Resources" and "Using Digital Controlled Impedance" sections.

2. In the case of an unused bank, VCCO should be tied to VCCAUX or any available VCCO.
(Do not leave it floating or connected to Ground.)