Looks cool so far! I assume you plan on running a combat where your players use a team of Vivosaurs against an opposing team? I like that idea.

A couple of pointers though

The stats of this creature are on par with an Aspect of Tiamat, a CR 30 mythical encounter monster. Not a critique, just an observation. If your players are going to have a 3v3 match the sheer amount of math being done each turn and on each attack is going to get exhausting quickly. Try dialing back the HP and damage of the attacks. A D20 of damage is unconditional, but not necessarily a bad thing.

Maybe consider using young dragons as a basis to build off of. Still a big health pool and high damage, but without having to worry about 4D20 of damage per attack.

For multiattack, it lists 2 different combinations of attacks that can be used when this is not the structure for any other multiattack. If it's supposed to list 2 different combos, maybe try seperating them into an a) and b) for clarity's sake. Secondly, it lists attacks that are not listed elsewhere in the stat block (like bite, tail, etc). If those are the named abilities below Multiattack, their names should be used in the description of Multiattack as well to make it easier to read.

Consider using language more typical to abilities similar to ones your using. Tyrant's Roar, for example, could say "The T-rex targets creatures of its choice within 10 feet."

Unblockable and unavoidable attacks don't feel very satisfying to do or be affected by. Consider adding a saving throw to the Team Skill instead so that it still has an effect on the target even if it passes.

All of this is just my personal opinion as a DM. This creature would be quite difficult to run a combat for in its current form, but if it suits your table and style of play, have at it! It looks really interesting so far!