I just finished another war in Vox Populi, as Japan finally capitulated after some 50-70 turns of conflict. I thought I'd share the story to give you an idea of how differently these things tend to play out when the AI is actually competent. First, a screenshot of the state of things immediately after the war:
So, first of all, the attack was not a surprise, as Nobunaga had attacked a few times earlier. He was friendly in the early game, but backstabbed me. Fortunately for me, my empire in the east was protected by the mountain range in the middle of the continent, and the fact that Lhasa and Panama City acted as a buffer between us. This, combined with the fact that the Shoshone gets a combat bonus in friendly territory, that I was in control of Mt Kilimanjaro, and that I had the Great Wall, meant I had an unusually easy time defending. However, striking back is an entirely different matter, so the previous confrontations had all ended in white peace. Now, for the latest, grander war, I wanted to find a more permanent solution. I knew that Nobunaga was planning another attack, as he was investing heavily in gaining the favour of Lhasa and Panama City. This would have given him much easier access to my lands as well as a good number of allied military units. Being aware of the threat, I was also pumping out diplomatic units and trying to solve their quests, as once war was declared, you can't really flip city states back. Fortunately, I had the city states allied to me when he finally declared war, and especially Lhasa did an excellent job as a first line of defense. I sent in my own units to support them, and together, we were holding off Nobunagas land forces. I was also helped by my highly upgraded Recon unit, who had extended vision and the ability to traverse mountains. He later became a casualty of war, but in the early stages he was invaluable for reconnaisance and harrassment.
After a while, Nobunaga offered me a peace treaty, but this time, I had different plans. I wanted to take Kagoshima, and maybe force capitulation, which would make him my vassal. It would also fullfil a city state quest and provide an XP boost to all my units. However, there were three main issues with this:
1. The geography, which had protected me, was doing an equally good job protecting Nobunaga. Anything I tried to send through the narrow pass northwest of Lhasa would be killed off very quickly.
2. My military forces were not nearly strong or advanced enough to push an assault without taking terrible losses.
3. Nobunaga had his Samurai units, which were really hard to kill, while easily destroying my units. Our tech levels were very similar, but his unique unit was in pgving him a definite edge.
To combat this, I did three things, which came together at nearly the same time:
1. Push for Gunpowder to get access to cannons and tercios. This would even out our unit strengths, and give me something which could survive a turn or two through the pass.
2. Build up a naval force in the north, which could support the assault, as well as a few landing troops to help gain a foothold.
3. Win the "Treasure Fleet" shared project. VP has a few more of these than the base game. Treasure fleet gives three rewards: a frigate (which no-one even had the tech for yet) for bronze, and admiral for silver, and a Grand Canal wonder in the capital for gold, which would give a small combat strength and sight boost for all naval units.
I immediately turned all my production towards Treasure Fleet, and managed to get all three rewards. Nobunaga and several others all managed to get the frigate and admiral. It also appeared that Nobunaga had noticed my naval buildup, as he had started to pump out ships himself. Still, I decided the timing was as good as it was going to get, and pushed the assault. My ships moved in from the north, and the frigate and some galeass helped me make room for the landing troops. I pushed a tercio through the pass, with cannons following close behind. My explorer was striking from the mountains. I lost quite a few units, but managed to finally take Kagoshima with a reasonable portion of my fighting force still alive. It was not quite enough to make Nobunaga capitulate though, so I set my eyes on Osaka. It was around this time I noticed an Assyrian naval force approaching from the southwest. I had some vessels there, doing some harassment and picking off the odd wounded unit moving too close to the coast, but pulled them back to let my unexpected help do their thing. To my joy, Nobunaga redeployed parts of his army to deal with the new invasion, which made it much easier for me to take Osaka. With Osaka in my control, Nobunaga finally capitulated, and he is now my vassal. I have lost a lot of units, but it was worth it. It is now time to rebuild, and especially to build up a navy capable of repelling future invasions. I also need to try and gain a tech advantage for the late game. In vanilla Civ 5 I would be leading by 10-20 techs at this point, but in VP, the aI usually leads by 1-3 techs in the mid-game. This time I'm already on par with them, though, so it's looking promising.
So that's how you can deal with the AI in VP...on Prince difficulty. I did win, but it took a concentrated effort, and I took serious losses along the way.