Why VDDIO is more than VDDcore supply in VLSI/ integrated chips?
I have seen VDDIO supply is more than VDDCore. 


This is required, as the smallest transistors (and other structures) in modern chip technology cannot support higher voltages than VDDCore limits. These structures are so tiny that they would break when supplied with VDDIO. But you need these tiny transistors in the core because they are the "fastest" and consume the least amount of chip area.

The higher VDDIO is usually required to be able to connect to some standard busses (say RAM or mass storage). The internal chip structures around the IO-pins are designed for these voltages and are thus quite a bit larger compared to the core.