Don’t know huge amounts about the culture; but think the Vorsook have an edge with the portals and mind readers. But I think the Culture have an edge in terms of sheer territory and ability to bring forces to bear; the Vorsook are more infiltrator and low-investment per win. And seem to treat Conquest as a game.
The Culture would probably go all in to remove them as a threat to prevent them causing long term damage over time. So I think the Culture have the edge.