"If I go through first, the portal might close on you," he says, reasonably. "Do you want to get portal-chopped?"

No one wants to get portal-chopped. Though Leo probably could hold it open long enough for V to get through if he went through first... but it's safer this way.

Besides, he has no idea if anyone in this universe knows how to portal-jack. It's just better all around if V goes first.