Starting with 2013.2, Vivado Synthesis supports the safe implementation feature via HDL.
As part of this support by the tool, you can set the safe implementation state to either a reset or "power on" state.
This will tell Vivado Synthesis to use reset_state as a safe state.
If the state machine does not have reset_state or power_on_state, then the safe state will not be implemented.
At present, these are the only safe implementation states supported by the tool.
In future Vivado Synthesis releases, user-specified states will be supported.