By the time of Uyghur Khaghanate and the apogeum of Tibetan power Goguryeo was already destroyed, while Silla was happily relegated to a vassal state status. Uyghurs on the other hand, while accepting nominal suzerainty of Tang, were extorting huge amounts of money and silk in exchange for military support or ceasation of raiding. Keeping them happy was paramount for Tang. Tibetans also were able to invade as far as the capital of Tang.Both Goguryeo and Silla built forces to match Tang in battle as well. Goguryeo in particular held loyalties to the tributary chiefdom to the Mohe tribes above the Yalu river and held a complex network of fortresses just to defend against Chinese aggression. Sillans also fought against the Tang in 670-676. Wouldn't the protocols be more diplomatic in nature?
It also has to be remembered that Tang of 8th century and Tang of 7th century are two different beast. The latter was still an expansionist power, while the former was mostly focused on keeping the borders protected.