AI is ridicolous. This is the main reason why i never play VH campaign. I like a challenge, but it's not fun having every nation attack you, once you share a border..

I had a game as Hayasdan, was locked in a war with the seles, who were also fighting the Ptollies and Parthians.
Once i shared a border with the Ptollies, they immediatly made peace with the Seleucids and focused on me, and on top of that the Parthians, who i didn't share a border with, was allied to and paid tribute to, marched an army around the caspian sea, to attack me...

That was the last game on VH i ever played, was simply too insane for me to actually bother with.

However, I should mention, that in another game (on hard), I stayed allied to Pontos, even though we had a long border (they conquerored mikra asia without my help, was kinda shocking), for some 60 years (campaign is on pause), without ever bribing them anything. I was, for once, pleasantly surprised.