Per [H]'s own review:
https://www.hardocp.com/article/2017/11/02/msi_geforce_gtx_1070_ti_gaming_titanium_review/1
there was literally one game where a Vega 56 outperformed the 1070 Ti at stock speeds. When the 1070 Ti was overclocked it out-classed the Vega 64 at stock speeds.
They're mostly equal in Vulcan and dx12, 1070ti faster in dx11, but the 56 is $50 cheaper and OCs to faster than a stock vega 64 as well. I'd probably buy a 1080 over a Vega 64 if I were in the $500 price range, but no way I'd buy a 1070ti for $50 more than a vega 56. If they were the same price, sure, but they're not.