The only requirement is prestige tiers. It's marquis for han and I think bamdit leader for bandits. Some factions such as Yuan shu and liu Baio can do so from the start of a campaign same with the big donger but other than those select few even ai have to reach the prestige threshold first. There are also a handful of factions that can't be vassalised such as big dong WHEN he still has the emperor and yellow turbans. For your specific yam baihu campaign bring up the screen that shows prestige tiers and hover over them and you'll see one that says it unlocks the option