Thats actually exactly what i did do. I vassalized whomever didn't change to state religion even when Pleased and killed some civs like Korea and Sri Vijaya. I ended up killing the Lombards because they wouldn't convert or vassalize for some reason. In the ended I got 8/7 which is why the arabs ruined the victory for me because they killed the Vandals which dropped it to 7/7. I was able to salvage things by reloading a save and letting all my african cities get captured and just focusing on protecting the vandals. But I think it would have been easier if I vassalized Korea and the Kalinka respawn and just spread Christianity to one of their cities (as they usually had only 1 or 2 at that point) but I didn't start using spys to vassalize until midway though my Byzantium game.My strategy for the Byzantines is :
Get at least 4 vassals and spread Christianity to their cities, then ask them to adapt state religion and Christianity by offering them techs as gifts!
Then let the massacre begin! Build an army of your UU and some heavy catapults and some lancers and roam around the map destroying every civ that is not a christian until you meet the requirements!
You will need at least half of the civs alive + 1 to be Christians to win, so It is easier if there are less civs around
Also as the visigoths the UHV for having 70 piety, 5,000 culture and being stable is checked already at the beginning even though I definitely don't have those first two. And I'm not aware of any way to check piety, I don't see it on the religion or victory screens.
And the Chinese Civil War around 214AD seems to be making the game crash. I'm not sure if it's just me or the save but I can't reach 214AD for some reason as Axum. Latest revision, 209. I also tried with an earlier revision and it still crashed. Is there anything I can do? I reloaded an old Kushan save and it happened again at 214AD during the civil war.
...and yet again theres another problem. The first UHV (5000 gold) for Vandals won't be completed even though I had 5000 before 500AD and around 6000 at 500AD.