In this paper, we present a partitioning, mapping, routing and interface optimization framework for energy-efficient voltage-frequency island (VFI) based networks-on-chip. Unlike the recent work that performs tile partitioning only with voltage-frequency assignment for a given mesh network layout, our framework consists of three key VFI-aware components, i.e., VFI-aware core partitioning with voltage and frequency assignment, VFI-aware mapping, and VFI-aware routing path allocation. In addition, we develop a VFI interface and its insertion algorithm to easily satisfy performance constraints. Our methodology makes cores using the same voltage and frequency unified to single VFI. Thus, our technique considerably reduces VFI overheads such as a mixed clock first input, first output buffer and a voltage level converter up to 82% and energy consumption up to 10% compared with the state-of-the-art work. It proves that our global energy optimization framework achieves better power-performance trade-offs.