Avoiding Realm Divide?

[Maltz]

I hope you have a save file just before triggering Realm Divide. Making an alliance with Uesugi and ask them to break up with Ikko-Ikki. This would make your life 10 times easier. Whether to make vassal or not is a minor difference compared to grand strategy. Take trading as an example, if you can remain trade with Uesugi, it would be a huge boost of your trade income. (10 provinces give about 4000 koku per turn.) Vassals are usually small. At most you can only make 1000-1500 koku from their trade and tribute combined.