In my current game noticed Ayutthaya had colonized Laos, and Dai Viet Cambodia, which was very nice to see.
My pet theory about this is that China has a lot of money to throw around and so ends up allying/vassalizing/annexing Ayutthaya and Dai Viet more often than not. I have yet to see Dai Viet push into southern China or conquer Ayutthaya, although very very frequently China seizes north Vietnam (Hanoi and Thanh Hoa) leaving only south Vietnam.