You may want to connect two separate 8-pin cables to your Vega. There may not be enough power coming out of each cable to drive the Vega 56, so the least you can do is to split the load between the two.
I do have 2 separate 8-pin connectors hooked up to it, have tested the
graphics card on 2 amd builds, 2nd computer used a 480. Only crashed out
the game and had to be closed with task manager to continue using the