In Chapter 795, we jump from Dressrosa to the aftermath of Sanji's side boarding Big Mom's ship.
What chapters did this happen? It's not specifically listed in the Manga Story Arcs in the arcs up to and including the Dressrosa Saga.
Edit: Revoked the correct answer because I have a new doubt. In Chapter 807, Usopp says that the last time he spoke with the Sanji half of the crew was when they evaded Big Mom's ship, meaning they had a chance to speak after evading Big Mom but before meeting on Zou. Thus, it couldn't have been skipped, because the whole crew, or at least Usopp, was there with them.