I have two VEGA-64 cards that can 't work on such high settings.
Settings of these cards:
GPU 1032 Mhz 905 mV. Memory: 1020 Mhz, 905 mV.
Consumes 170W GPU power.
0,97 g/s
Use AMD Mem Tweaker to apply weak settings on Vega Cards.
That should provide a bit boost to the hash rate.
Also, undervolt the core more as undervolting memory doesn't do anything.