Having piss-poor prestige helps a lot in becoming the junior member of a PU. Just try as hard as you can to get your king killed and hope that you get a lucky hunting accident. Also marry any and every high-prestige country you can.
Junior PU members do, in my experience, keep their vassals but it leads to the laughable sitution where Country X declares war on the junior partner's vassal, the junior partner calls in the senior PU partner but the junior partner remains the war leader unable to make peace until the senior partner peaces out of the war.
It's a complicated affair but yes, better than being a vassal and having half of your monthly tax come straight out of your coffers. Not out of your tech money but staight out of your pocket!
Junior PU members do, in my experience, keep their vassals but it leads to the laughable sitution where Country X declares war on the junior partner's vassal, the junior partner calls in the senior PU partner but the junior partner remains the war leader unable to make peace until the senior partner peaces out of the war.
It's a complicated affair but yes, better than being a vassal and having half of your monthly tax come straight out of your coffers. Not out of your tech money but staight out of your pocket!