Avoiding Realm Divide?

[Jacobin]

As Chokosabe I triggered RD when I was in almost exactly the same position as in Maltz's map - but had the Urakami as vassals up in the NW of the main island - and they stayed loyal for a long time afterwards - possibly because they only had a couple of poor provinces and could not afford to raise a full stack?

However even though I had taken every economic upgrade available the loss of trading partners (other than the Urakami I was at war with everyone) proved crippling and I could only afford one killer stack of samurai and warrior monks - while the Uesugi and Ikko Ikki who owned the east and centre of the map allied and got together a vast horde of six or seven full or half stacks and systematically used it to besiege and starve into submission each of my fortresses while their Ninjas run riot so that I had to spend most of my money repairing acts of sabotage rather than raising new armies.

Next go I will make a point of making no vassals until after RD and then vassalise every province that's not vital.

[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.

[Fagar]

Next go I will make a point of making no vassals until after RD and then vassalise every province that's not vital.

Yes a newly raised Vassal has just the one province and trade income from each one is very minimal..(often less than your own farm income from it would be)
I never really make vassals after RD only before as a tool to delay or stave off RD until I am in a better position..

Much better to attempt to sway another strong party to come along with you for the ride through RD..
Just before you cop RD is the time to forge these alliances to take advantage of the full new +100 for military alliance and if you have had them as trading partners for a long time that is even better as there will be substantial +'s here as well..
Throw in a declare war on one of their enemies on their behalf with no asking for money and you will get a hefty +'s for gifts and who cares about a new enemy anyway if you are just about at RD ;-)..it is grabbing this one substantial ally that counts (in your case Uesugi)
+100 is also up for grabs if you are in a position to ransom off one of your family, gives you 8 years of slowly diminishing +'s...
All of these things to me are far more important than the use of vassals...

[Jacobin]

My current Daimyo has a whole bunch of small children and I've never tried hostage exchanging so that might well resolve another problem I am having with being attacked by three big factions simultaneously (plus my only allies are the Ikko Ikki who really can't be trusted....)