It occurred to me that G.fast could possibly use the VDSL2 frequencies in a similar manner to how VDSL2 uses the ADSL2+ frequencies. As in, G.fast downstream on the VDSL2 downstream bands, and G.fast upstream on the VDSL2 upstream bands. Although the on/off nature of the G.fast TDD transmissions might still pose a problem for VDSL2.
At first, I thought the reuse would be like the way VDSL2 has power masks for ADSL2+ ... when further from the exchange, the higher ADSL2+ frequencies can be re-allocated to VDSL2.
But a few extra seconds of thought made me realise that the same dropoff in usage wouldn't happen for G.Fast, when being targeted close to the cabinet.
Your suggestion makes sense, though. NEXT would likely be catastrophic, so restricting G.Fast to existing directions would make sense.
On top of this, G.Fast adds discontinuous transmission functionality, so that it only transmits when needed. If they could make use of that feature such that the VDSL2 frequencies only got used when all the other tones were full, that would minimise interference.