What do you get for winning a Vassalization war in CK3?

The defender becomes the attacker's Vassal (patch 1.20.0.3).

What does the Vassalization casus belli cost?

piety and prestige; the amount depends on the titles, claims and distance, worked out when you declare.

Casus belli · Vassalization

Vassalization

Vassalization is a casus belli in Crusader Kings III (vassalization). If the attacker wins: The defender becomes the attacker's Vassal. If the defender wins: The defender remains independent. Declaring it costs piety and prestige. The war goal is worth 80% war score. Patch 1.20.0.3, from game/common/casus_belli_types/00_vassalization.txt.

Quick answers (3)

What do you get for winning a Vassalization war in CK3?: The defender becomes the attacker's Vassal (patch 1.20.0.3).
What happens on a white peace?: The defender remains independent.
What does the Vassalization casus belli cost?: piety and prestige; the amount depends on the titles, claims and distance, worked out when you declare.

How the war ends

Attacker wins

The defender becomes the attacker's Vassal.

White peace

The defender remains independent.

Defender wins

The defender remains independent.

If the war becomes invalid

The … has ended: the defender can no longer be vassalized.

Who can declare it

The game shows no text of its own for these conditions; the script is below.

Every vassalization war (the group's allowed_for_character) (7 lines of game script)

herders_and_tributary_constraints = yes
pam_personal_pacifism_offensive_war_ban_trigger = yes
tgp_dynastic_cycle_offensive_wars_ban_trigger = yes
tgp_war_ban_all_ministers_trigger = yes
NOT = {
	government_has_flag = government_is_theocracy
}

As written in game/common/casus_belli_groups/00_casus_belli_groups.txt.

Attacker (allowed_for_character) (20 lines of game script)

OR = {
	has_perk = forced_vassalage_perk
	any_character_struggle = {
		involvement = involved
		has_struggle_phase_parameter = unlocks_forced_vassalization_casus_belli
	}
	AND = {
		OR = {
			has_game_rule = default_conquerors_bonuses
			has_game_rule = extreme_conquerors_bonuses
			has_game_rule = random_conquerors_bonuses
			has_game_rule = reduced_random_conquerors_bonuses
		}
		has_variable = conqueror
	}
	ai_should_get_conqueror_bonuses = yes
}
NOT = {
	government_has_flag = government_is_landless_adventurer
}

As written in game/common/casus_belli_types/00_vassalization.txt.

Defender (allowed_against_character) (40 lines of game script)

scope:defender = {
	top_liege = this
	trigger_if = {
		limit = {
			scope:attacker = {
				is_ai = no
			}
		}
		scope:attacker = {
			any_neighboring_top_liege_realm_owner = {
				this = scope:defender
			}
		}
	}
	trigger_if = {
		limit = {
			NOR = {
				scope:attacker = {
					has_perk = forced_vassalage_perk
				}
				scope:attacker = {
					ai_should_get_conqueror_bonuses = yes
				}
			}
		}
		any_character_struggle = {
			involvement = involved
		}
	}
	NOR = {
		any_held_title = {
			is_holy_order = yes
		}
		AND = {
			exists = faith.religious_head
			faith.religious_head = root
		}
	}
}
scope:attacker.primary_title.tier > scope:defender.primary_title.tier

As written in game/common/casus_belli_types/00_vassalization.txt.

Cost to declare

Costs piety and prestige. The amount is worked out when you declare (by the titles targeted, claims, distance and more), so the script is shown as written:

Cost (19 lines of game script)

piety = {
	value = 0
	add = common_cb_impious_piety_cost
}
prestige = {
	value = 0
	add = {
		desc = CB_SIZE_OF_TARGET_REALM_COST
		scope:defender = {
			every_sub_realm_county = {
				add = {
					value = 125
				}
			}
		}
	}
	multiply = vassalization_cb_prestige_cost_mulitiplier
	multiply = common_cb_prestige_cost_multiplier
}

As written in game/common/casus_belli_types/00_vassalization.txt.

Source: game/common/casus_belli_types/00_vassalization.txt and the English localisation, Crusader Kings III 1.20.0.3. “The attacker”, “the defender” stand where the game names the character; “…” marks other words it fills in during play.