What is your design size?
You can convert your design after synthesis to HSPICE netlist automatically using Cadence IC6.1. Once you get the netlist, simply change the VDD and simulate it. If your design has more than 10,000 transistors, the HSPICE can take a long time.
Alternatively, you can make new library for synthesis/map with modified delays obtained from individually simulating cells in the library for different VDDs. The latter solution is a bit harder.
Mehrdad