First of all, any vassals you make before you become shogun will be hit with the realm divide diplomacy penalty once realm divide starts. If you wait until after you become shogun to make vassals, then they get a much milder penalty against you, and there's still a chance that they will eventually turn on you.

Personally I do not think that vassals are ever worth it at all. Before realm divide there's always a decent chance that they will turn on you, and if they don't then there's a good chance that they'll take a bunch of land which triggers realm divide way earlier than you want it to, because their land counts as your land.

Yes you get a small money payment from the vassal and a captive trade partner but it's not worth it when the vassal can completely upend your campaign at any time.

The only time I will ever make a vassal is if I am at war with clan A, and have defeated their last army, and clan B has just declared war on me. I make clan A my vassal so they at least stay off my back for a while, and then I go deal with clan B. But I do this knowing that clan A will eventually turn on me or get taken over by another clan.

But usually I will just make peace with clan A and not make them my vassal, because clan A is so weak that they'll probably be taken over by another clan.

Once you are shogun then it's safer to make vassals but as I said there is still a good chance that they will turn on you. I would rather control their land and not have to worry about being backstabbed when I need to deal with the other clans on the map. Once I am shogun I have plenty of money so I don't need payments from vassals or trade with them. Someone will probably come in here and say that you can just pay your vassals massive amounts of money after realm divide to keep them happy with you so you can trade with them but that is surely not worth it.