just reading the last few comments made me realize how hard it will be to go back to vanilla bnw.
I completely understand your concerns about vanilla AI being poor. But RFC is very different from normal civ because the conditions in which each game is played are (more or less) exactly the same. So AI problems can be tweaked far more easily than would be possible for a normal vanilla game. A lot of things in RFC are hardcoded, for example:
1. Time-specific barbarian spawns, such as for Rome in the 5th century AD or Seljuks horses in the middle east in the 11th century AD, etc. So if we find by playtesting that players are easily beating the Barbarian invasions as Rome, we could just add more Barbarian units/increase their strength. Having a smarter tactical AI would be ideal, but in its absence, this approach works as well.
2. AI tech preferences are, while not entirely predetermined, heavily skewed towards certain sides of the tech tree, and beeline specific wonders. Spain and England tech faster towards compass and associated techs and China beelines the Great Wall, for instance. This circumvents the AI making stupid tech/policy choices.
3. Different AI's also get a variety of specific tech and production bonuses, such as England and the Netherlands to simulate their rise as technologically advanced civs despite having relatively small empires (before they established their colonies).
As regards balance, I don't think there is any good reason Rhye can't pick particular changes from VP that he likes and implement them in Catapult. In particular, I think a good fit might be how the Exploration tree has been renamed to Imperialism, with appropriate changes (rather than just buffing coastal cities, Imperialism boosts tile yields, GG/GA yields and puppet city yields). Fits in very well thematically with a colonial power like Spain or England.