It is always a good idea to read the forums. Diplomacy on VH is borked. On VH it is pretty much the Human vs all AI neighbors by mid game. The only factions you can keep as ally's are ones you have no border with when playing on VH. The AI also cheats with considerable amounts of cash for the AI factions on VH. At some point the AI factions will attack if you have a common border. Even with no provocation, and when it is obvious they will get their butt kicked