A 2070 will be somewhat faster for gaming than a Vega 56, or a Vega 64 for that matter. If you are considering a 2070 vs Vega 56, you might also have a look at the 2060, which should be coming out next week, and based on reviews its average performance should be roughly in line with a Vega 64 or a 1070 Ti, and only around 15-20% behind a 2070, at what should be a notably lower price. The 1070 Ti might be another worthwhile option if you can find one at a price significantly lower than a 2060, otherwise I would go with the newer 20-series architecture, as it has some features that could potentially offer better performance and visuals in future titles.
Your CPU should not significantly limit the performance of any of those cards in today's games. What resolution and refresh rate is your monitor though?