What is the range of voltage that can be fed to VCC_CONFIG?
In the Virtex-4 User Guide (v1.2), page 283, it says that the dedicated I/O pins (VCC_CONFIG) are restricted to 2.5V only.
The IOSTANDARD of VCC_CONFIG is actually LVCMOS_12_F. All dedicated input pins operate at the VCC_CONFIG LVCMOS level. All active dedicated output pins operate at the VCC_CONFIG voltage level with the output standard set to LVCMOS_12F. Therefore, VCC_CONFIG can be fed with any voltage level ranging from 3.3V to 1.2V. The Virtex-4 User Guide is incorrect and will be updated in the future version.
Also, see (Xilinx Answer 20737).