I hear it a lot, but I never agree with it: diplomacy is not non-existent on VH campaign difficulty - it's just very hard!

you have to put a lot of effort into your relations with any faction you want to remain allied with, and realise that regardless what you do, on any campaign difficulty, sooner or later if you share a border with a faction and you are expanding and getting more powerful, they will opportunistically attack.

sure, this is not always sensible, but this happens on all campaign difficulties

I recently posted a screenshot of my venetian campaign on another thread and someone said they couldn't believe the campaign map had three major powers besides my own, and I was allied to all three in year 1480 or something. that's because I spent the whole game keeping them sweet and making sure my relations were always high

diplomacy works on VH, it's just very very hard!