Crossfire doesn't double the vram, it's still just 1Gb total. So with the integrated graphics, you'll use partial system ram, which can be set for more than 1Gb. Crossfire only works on supported DX10/11 games, it has no support on DX12 games, and is dependent on the amount of support. Some games its ok, more games its slightly miserable, some games its so dismal you get better results using a single card.
Best bet will more than likely be running a single 5750 for the majority of games, only a few will show any real benefits from CF.
The Vega integrated graphics are pretty good, but won't top a dedicated card like the 5750 for gaming.