When fighting a dragon: it and players roll a 1 for initiative, no lair actions can be used in the fight?

Could a Blink Dog be the size of a Chihuahua and still be considered a medium creature?

I have a player with a 22 passive perception. Do I even need to make him roll?

Vulnerabilities I don’t know a single creature or race that has one?

Got a player who halts gameplay… Any advice?

Would you ever consider posting the stats of the Vonindod?