Do you take vassals or don't?
(A little longer: Getting closer to domination victory, quite tedius on marathon and large. Just qonquerres 50% of Joao...and he capitulate. Should i accept, or should i keep going and take the rest also?.How about early in the game, do you take vassals that ask nicecly?)
As with much about this game, it depends. What are you trying to achieve? If Domination, you should consider that a vassal counts toward 50% of your land requirement. So if you are just trying to win DOM as fast as possible that is something to consider. Maybe capping Joao right now will win you the game right now...not sure since I don't see the game. But other things to consider is the time cities take to come out of revolt and expand borders vs. a vassals land not having that issue. In fact, you might just give back some or all of his cities. So you could cap him now and just move on to the next target...or cap now and win..again not sure.
Otherwise, my main consideration on vassals is Space vs. something else. Generally, optimal Space games means more land..land that you own. However, a good vassal or two can be nice as well. Vassals that have a sustainable economy and can tech stuff for you (you can control what a vassal techs via the dialogue by the way). So when conquering a foe and determining whether to vassal someone or take their land, I ask myself a) is that land better under my control b) is the vassal viable, i.e., will he contribute to my space game. If the latter, I will usually make sure that vassal has some decent land and cities - probably gifting back some cities that I took.
For Conquest, vassals can make conquest faster simply as you cap them as soon as you can and move on to the next target, as opposed to just grinding out the rest of their cities. Some with Domination to an extent, but one has to consider the land percentages that factor in there. Note that if taking cities that would be burdensome one way or another, you can always just have some settlers build and settle them near the end.
For Diplo, we use a term Diplomation..that is going toward a Diplo win via a Domination route. In other words, take some vassals as they will vote for you.
Vassals pretty irrelevant for Culture games, as a culture game is generally very focused from Turn 0 and combat is usually avoid except maybe very early to grab some good cities nearby.
As to you last question, which appears to relate to what we call "Peace Vassals" or AIs that voluntarily capitulate, I generally always accept a peace vassal unless I already had specific designs are their land and wonders.
Lastly, one thing I do consider in the fairly early parts of the game with my first target or two, is likely just taking all their land since it can be put to good use since it is earlier in the game. That is, more time to develop the land and get infra in the cities. Also, makes later wars and capitulations easier. However, that may depend on how well that particular war is going...sometimes I just want out and capping is the better option. Of course, the higher difficulty one plays the harder these wars become.