I always thought the Don saying "is it necicery" meant that he knew what Luca had to say I doubt he was scared to be in a room with Luca.
Right, Vito was not scared of Luca. But as others mentioned Luca was the only person who could make Vito nervous. Considering Vito's own fearsome reputation that says a lot about Luca..
On Connie's wedding day perhaps Vito just didn't feel like taking all the energy needed to show Luca that he valued and respected him. The Don was trying to have positive thoughts...
