Gonna weigh in on this given my experiences this last year.
A lot of these cons need to see why they benefit from having Vtubers there. Remember, they are spending money to have Vtubers there, so you gotta bring things that will generate interest!
A few suggestions that have worked for me and some friends:
Do a Vtuber Smash or Pass panel!
Have a design workshop for character/vtuber design!
Bring a standee for people to see/take pictures with!
It also helps to get to know con staffers, regulars, and go to closing ceremonies. Don't be afraid to put yourself out there!