User blog comment:The Will of Deez/The Curious Case of Donquixote Doflamingo/@comment-1595565-20150815194658/@comment-24769684-20150815220928

http://www.mangainn.me/manga/chapter/128325_one-piece-chapter-767/page_5

You know, I went and checked, thinking it had been common knowledge since 767 came out that Vergo and Trebol were at the shooting. Looking back, however, the panels showing them were actually foreshadowing their first meeting between them. My bad on that.

But Doffy did meet the executives before he shot his dad. In Chapter 782, we see that he was approached by them after he inadvertently used Haoshoku Haki on people trying to lynch him. Trebol gave Doffy his gun then, and him giving it to Doffy is foreshadowed in the top right panel of above link. Doflamingo then used that gun to shoot his dad. If you look later in 782, you see that the executives later pledged themselves to Doffy after he escaped from Mariejois. None of the executives were at the shooting, but we know that the scene here came first.