Vassals are really the means with which to keep huge numbers of troops and also to attack multiple targets at once. Each vassal can store 500 troops and once you send troops to your vassal you cannot recall them. Your vassal can use your troops for defence but hopefully they don’t use them all up on you.
When getting vassals it’s very important to space out your vassals in two ways.
Firstly get vassals all over your surrounding area and not ones just very close together as you can cover the maximum area and attack a lot of different castles, players and wolf lairs. Secondly space out your vassals evenly between all your cities. DO NOT have all your vassals connected to one city. Number one because if you do it will take forever to fill them up with troops. And secondly if you ever lose your city you will lose all your vassals.
Between 3 and 5 vassals is a good number to have for each city as you can fill them up relatively quick and if you lose a city it’s no big loss.