An AMD Vega 8 is an integrated graphics chip, it is not a dedicated graphics card. Seeking formal recognition for it is I'm afraid not going to do much of anything for the game except perhaps silencing the warning. The warning should only be appearing one time though, does it really return each time you launch the game?
There are limits as to how far we can take TS3 on integrated graphics and some of the heavier EPs (Pets, Seasons, possibly IP, ITF) might be too much for it to handle. Does the game play at all as it now is?
The 560X (not 580X or Vega) and I'm still on Mojave so I am certain I could get TS3 for Mac up and running if I wanted to. I will eventually update the macOS to Catalina and that will make running the game impossible as currently Catalina cannot run 32-bit applications (there is an update for this in the works by EA, due out later this year).
But I don't run the Mac version of the game, its built-in 2 GB RAM restriction is a game-killer for me. We'll see what the pending update brings us. I've got Bootcamp going and am running the Windows version of the game on Win 10 and it's all fine, if one can tolerate having two operating systems to manage. This is how I'd been playing TS3 for many years on my older 2011 iMac (except on Win 7 rather than 10) so there were no really big adjustments to take into account other than my now being able to enjoy the much stronger hardware.