This command will make the specified country tag a vassal of the second country tag.
vassalize [country tag] [country tag]
|Country Tag||The tag of the country to vassalize.|
|Country Tag||Optional. The tag of the country to do the 'vassalizing' - if not specified, this will be your current country.|
The above command would make England (ENG) a vassal of the country you're currently playing as.
vassalize HUN FRA
Hungary's country tag is HUN, Frances country tag is FRA (view all country tags). This command would make Hungary a vassal of France.