We have no idea about Vega, but it will bottleneck a 1070 some, but not too much. There are a lot of factors, if you are playing at 1080p, it will bottleneck it more. If you are playing at 2k, less. Some games are CPU heavy such as battlefield one. Some games don't require much of a CPU at all. So for the games that are light on the CPU, your bottleneck will be less and for heavy CPU games it will be more.
For a gaming rig, you want to spend most of your money on a GPU. While the CPU is important to a degree, it is not nearly as important as the GPU.