Really short summary:
Nobody can tell you if any of the Vega RX cards will be comparable to a 1080Ti because nobody knows. It's probably worth waiting because information should be available late July or very early August.
Short summary:
AMD released the Vega Frontier Edition, which is meant for low-level development and research work. It's not pro kit because it doesn't have the certified drivers that requires, but it's not home kit either. Bit of a niche market, but it is a real market. It has a gaming mode and AMD lightly mentioned it mainly in the context of someone who wants 1 machine to both do game development on and game on.
People bought them and reviewed them for gaming. They were mediocre - between a 1070 and a 1080 but using far more power and generating far more heat, so much noisier and with little or no scope for overclocking. Also, they're over £1000.
AMD have mostly ignored that, but have replied saying (truthfully) that Vega FE isn't really meant for gaming and a different version - Vega RX - will be released soon that is meant for gaming.
There is a lot of speculation about how much difference will exist between Vega FE and Vega RX. A lot of speculation...and almost no information. The GPU will be the same, but it is possible that the FE drivers don't efficiently use it for gaming. Maybe RX will be far better for gaming than FE. Maybe not. Maybe it will be significantly cheaper than nVidia cards with similar performance. Maybe not. Maybe it will use much less power than FE. Maybe not.