I play on vh/vh and tho normally its extremely hard, i find its farely easy to turn factions with poor starting positions.For example i almost always turn scotland into a vessel when im playing with France or spain.Or when im playing as a catholic faction its farely easy to get the egytians to succumbe to slavery once ive got them down to one city.If i blitz factions that also helps,i guess the ai doesnt anticipate the multitude of a mass and a effective invasion and once youve quickly taking two of their cities,offer them their cities back in exchange for them to be your vassals.