I needed three more cities to win as the Mayans so I went after a six-city kingdom already at war with my allies. They defeated my army in the only battle we had but then sent am emissary asking for a cease fire. Since I never had the AI accept a vassal state and I was going to attack again regardless, I asked them to become a vassal. To my surprise, they agreed and immediately, I got a victory screen. In the past, being a vassal was merely becoming allies with trade rights but apparently, this was corrected to them actually counting towards your city tally (and, I assume, visa-versa). Yeah for CA.