Regions and Vassals
Rob went over some basics of Regions and Vassals in our last Developer Diary, here (https://forum.paradoxplaza.com/forum/developer-diary/developer-diary-world-map.1600764/). Regions define a big chunk of your Nation. They have territory and population, provide places for Improvements and Capital Buildings to be built, and they are the primary source of the Goods and resources that power everything you do. Vassals are similar to Regions, but they are not directly controlled. Vassals grow on their own, expand borders, build buildings, and so on. They also pay a portion of their income to their Nation, which can make them quite valuable. When you conquer an enemy Capital or use a Settler, you gain a new Vassal. Your Vassals have an Integration level, which describes how much the people there feel like part of your Nation. Vassals you gain by using a Settler need less Integration than Vassals you acquire after your army takes control of the place by force, but Integration in either will slowly increase over time (and a power in the Diplomacy Domain can be used to speed this up).
Minor Nations
Another way you might add a Vassal to your Nation is through a Minor Nation.
Governments
One factor that can have a substantial impact on the number of Regions and Vassals best for a Nation is Government. All Nations have a Government, starting with Tribal. In Ages 3, 5, and 8, other options become available. Structurally, Governments are similar to National Spirits – they provide access to a set of Ideals that can be unlocked using Government Domain Points. Unlike National Spirits, they do not stack. You only have one Government at a time and taking a new one replaces the old one. Governments provide a specific Government Building and also define the baseline value of a Capital City. The Capital of a Nation with the Tribal Government, for example, generates 2 Culture per turn (among other things). As a Democratic Republic, the same Capital City would provide 4 Culture per turn.

Diplomatic Relationships
Once they have made contact, Nations have diplomatic standing with one another. There are five diplomatic states: War, Hostilities, Peace, Open Borders, and Alliance. Nations begin “At Peace” with one another and the simplest form of diplomacy is available immediately – you can propose improving your diplomatic state (for example, suggesting going from Peace to Open Borders) or you can declare a move in the other direction (for example, declaring that you’re changing from At Peace to Hostilities). More sophisticated diplomatic options are available if you deploy an Envoy at another Nation. With an Envoy in place, you gain a new set of actions that allow you to adjust your relationship in more specific ways (…like extorting money from weaker neighbors).