I don't really follow the UFC, so I don't really understand how popular Marlon Vera is but compared to McGregor people will be more aware of McGregor and it could be that his level of popularity makes Casino have to spend more to sign him become a brand ambassador, but it could be also that Roobet is targeting the Latin America market to be able to get a lot of players from there.
It's not just about UFC for that situation. Mcgregor is known wildly outside of UFC world as well, he is a TMZ level of fighter whose regular life becomes a story, his clothes, his jewelry, his actions, his purchases. Basically Mcgregor is known by so many people that he could become an action movie villian in Hollywood right now in a good level movie and wouldn't really be surprising.
This is why I think that Vela is not the same level, Vela is known in the UFC world a lot, and I mean A LOT, but only if you are in the UFC circle, not at Hollywood or global levels like that. Which isn't bad, it is still a good sponsorship one way or another.