Wait, you can have vassals as a republic? I thought the concept of vassalization was something a king should have.Win wars, gain prestige, expand sphere of influence. Takes a while, but that will raise your diplomatic ability enough to ensure they accept your annexation request. Also, convert to a republic and select a leader that has good diplomatic skills. And hire an ambassador. Anyway, size doesn't matter when it comes to annexing vassals. I've had 13 province Croatia at "Very Likely" on annexation request while OPM Montenegro was only at "Maybe".