Altera® devices support MultiVolt I/O pins which allows various combinations of VCCIO voltages and input signal voltages. Most device family handbooks have a MultiVolt I/O table in the Device I/O Features chapter. This table gives you a quick check box reference to see if the signaling voltages are compatible with the VCCIO voltage you are using. The compatibility is based on the I/O switching specifications from the Device Datasheet.
For example, an input pin powered by a 2.5V VCCIO has Vil and Vih specifications which are compatible with receiving 3.3V LVTTL Vol and Voh specifications.
The Device Datasheet shows the minimum Vil and maximum Vih voltages for each I/O standard based on JEDEC specifications and requirements. Altera devices are tolerant to the Vi (DC input voltage) as shown in the Recommended Operating Conditions for the respective device family. This Vi specification applies to the device I/O pins for all supported VCCIO voltages.
For example, the Vih maximum specification for a 2.5V standard is VCCIO 0.3V. Per the I/O Standard Specifications, 2.5V signaling should not exceed 2.8V for the maximum Vih. However, the Vi specification from the Recommended Operating Conditions section of the Datasheet supercede the maximum Vih from the I/O Standards Specifications. Device families that support 3.3V I/O standards will have a Vi maximum of at least 3.6V.
The Vih maximum specifications shown in the I/O Standards Specifications section of the Device Datasheet is reflective of the JEDEC requirements of the I/O standard and is a subset of the actual device tolerance. You can use the Vi specification from the Recommended Operating Conditions to know the actual device tolerance to input voltage levels.