I think there are two reasons. First, maybe Vito was afraid Luca would ask him to do something that he could not do, thus breaking the notion that no Sicilian could refuse a favort on the day of his daughter's wedding. This would incur Luca's wrath
and as is mentioned before Vito feared Luca cause he could not control him.

Second, he did not want to have to meet privately with Luca when he had so many other things to do.

I think he was relieved when Luca came to deliver the money for his daughter's wedding purse privately instead of in public.

Finally, I do not think Luca stayed for the rest of the party. He probably took those sandwiches that were thrown at Paulie and
went home.