Both are good articals. Overshoot and ringback are unavoidable when trying to balance edge rates and frequency.

The important take way from this is that VTT voltage is used for FSB signalling, and Vref is a DC voltage derived from VTT which is used by the agents on FSB to determine logic high or low values.

Adjusting Vref to it's optimal level can improve your voltage margin, and thus, better overclocking.