After watching those i decided to give the game a spin. So as one does when first playing Europa Universalis 4, i started as Japan.
(Okay, dealing with the diplomatic nightmare that i'm sure is Europe itself seemed like too much of a hassle. I wanted something relatively simple to deal with.)
So of course as my "first" game this involved learning a lot of things on the fly. Although from what i gather Japan plays rather differently from most other countries, so many of the things i've learned may not apply well to other situations.
The main unique thing about Japan is that it starts with a large number of vassals, all of the special Daimyo kind. Which means that despite technically being your vassals they can declare war on each other or on you at any point and there's fuck all you can do about it.
So very shortly after starting the game i accepted the "assimilate Takeda" mission (or whatever it's called.) He starts with just a single province so i figured that would be easy to take care of. I'd noticed that one of the diplomatic options was to assimilate vassals, so i figured i'd just get my relations up with Takeda and take over all peaceable like. It wasn't until after i'd already accepted the mission however that i realized there was one additional condition to that diplomatic option, they have to have been your vassal in game for 10 years.
Well that was annoying, but i figured i could just fast-forward through the first ten years and then start assimilating people. So i sat and waited. Daiymos started declaring war on each other and seizing territory. Then about two years before i was going to be able to take over Takeda someone declared war on him. And then kicked his ass. Then started sieging his only province.
After waiting all this time i didn't want to lose out on my mission, much less my intended first step in unification. So i tried enforcing a peace on the aggressors. They laughed in my face and i got dragged into the war on Takeda's side. Amusingly despite the fact no one was willing to listen to me my prestige was high enough that i got put in charge of the war. So i gathered my (rather small) army and tried to break the siege. And got my ass kicked. I think i reset and tried once or twice more, with pretty much the same results.
So then i decided i'd gone about this all wrong and went back to my first save, right near the start.
This time i played the waiting game again, but this time only until the first war broke out. Then i went a bit Machiavelli. One of the parties of the war, luckily part of the smaller defensive alliance, had territories right next to me. So i approached them and tried to enforce a peace. They laughed in my face and i "reluctantly" joined the alliance against them. And once again i was put in charge of the war because of my prestige/position. I then marched my army in and helped conquer them. Then for the peace negotiations i claimed their provinces for myself.
So i then just kept repeating that tactic. Wait for my Daimyos to declare war on each other, bully the weaker side, be "forced" to join the war, and take their stuff. In between the fighting i started diplomatically annexing other vassals who weren't getting involved in the wars or were too strong to easily take on in war.
I did run into one complication that ended up stalling me for awhile. You need to get relations up rather high to annex a vassal. Every time you annex a vassal you take a significant relation hit with all your other vassals. Pretty much the same way that conquering people makes the rest of your neighbors nervous. Except in this case it directly impacts your ability to annex those other vassals.
So i believe in the 60s or 70s i annexed my third and fourth vassals. Japan has two diplomats and it turns out that if you send them both out at once you can annex two vassals at the same time. You check for success at the start of the process and get the relation hit at the end, so it still works even if the relation hit from the first one ought to ruin your chances with the second one. However at that point i had a total of about -100 to my relations with my last two vassals. At the current improvement rate it was going to take almost 20 years for relations to recover enough for annexing to be viable again.
So just conquer them outright, simple no? Except in Europa Universalis you get a big penalty if you declare war without a casus belli. For those unfamiliar with the term, casus belli is latin for "diplomatic excuse to kick ass and take names." I hadn't needed that before because i was joing in on existing wars through diplomatic chicanery. Now however the remaining two Daimyos were not sharing a border and were probably far more concerned about me than they were about each other, so they refused to start a war on their own. And it turns out that although all the Daimyo have an automatic casus belli against both the other Daimyo and the Shogunate, the Shogunate does not get a casus belli against the Daimyo. There's a covert diplomatic action to manufacture a casus belli, except it turns out that you can't use that against your own vassals.
So i had to spend over a decade sitting with the three of us glaring at each other, waiting for tempers to cool enough to make annexation feasible. Meanwhile across the sea Manchu declared war on Korea and started taking their territory, which worried me quite a lot since Korea was my next logical step. Both for logistic reasons and because i had a mission ready which would grant me casus belli against Korea.
Luckily some good diplomatic events happened that shaved a couple years off the process and i was able to unify Japan while Korea still had about half its territory left. Since they'd taken a break from fighting i took the time to finish tidying up my territory and complete some domestic missions before declaring war on Korea.
Despite some initial stumbling blocks the war with Korea went quite well, except for one tactical diplomatic blunder on my part. The mission had given me casus belli to seize Korea's two southern provinces. When i finished sieging all of the provinces they had left and went to the peace talks i didn't quite have enough influence to demand all their territory. So i asked for and got the southern four or five of their provinces, leaved two left. I figured i'd just start another war later and grab those last two. Except as soon as i'd agreed to that i got a message saying i'd lost my casus belli. Because the entire pretext was i wanted the two southern provinces, and i had those now.
At this point i'm not sure if i should fabricate a claim on the rest of Korea or just get started with what seems like the inevitable war with Manchu, which probably isn't going to be very fun.
Short version: When consolidating vassals (at least as Japan) make sure to conquer as many as you can first, and only switch to annexing for the last 2-4. And if you have a casus belli based on seizing a limited number of provinces but you actually want to seize the whole country then make sure not to ask for all of the provinces your casus belli is based on until you can get all the rest of the territory with it.
So it's 1495 now, i've got Japan untied and have claimed approximately 1/3rd of Korea (With a little over 1/3rd currently belonging to Manchu.) I've gotten one Administrative tech, one Diplomatic tech, and two Military techs. I'm not sure whether or not that would be considered an okay pace by experienced players.