The 1080 Ti is flat out better than the Vega 64. The Vega 56 had an edge over the 1070, but they came out with the 1070 Ti to counteract it. The Vega cards are very rare and more expensive than they should be because of the Crypto craze, sadly.
If you do use your PC for mining, check what kind of GPU architecture your mining algorithm uses better. Currently, most use the Radeon architectures, which is why they are hard to find and cost too much.