The ISE 5.2isp1 design implementation software refers to the LVCMOS 2.5V I/O standard for Virtex-E as "LVCMOS2". Because the associated IBIS model is called "LVCMOS25", IBISWriter erroneously reports that a buffer model is not available. For Virtex-E, the LVCMOS25 buffer model should map to the LVCMOS2 I/O standard.
This problem is fixed in the latest 5.2i Service Pack, available at:
The first service pack containing the fix is 5.2i Service Pack 2.