It's definitely going to happen, there won't be that many arcs left after Elbaf, and the reunion happening at the very end would be too late.
Elbaf is the perfect arc for the Straw Hats and Vivi to reunite.
Also Vivi was part of the Little Garden arc which is of course the arc that introduced Dorry and Brogy, and also the arc in which Elbaf was first mentioned.
Morgans should be smart enough to know that the Straw Hats will be going to Elbaf after Egghead (the dude has a flying ship, he should know the courses for islands/navigation).
Morgans is obsessed with uncovering the truth and then twisting it, he knows the official Egghead narrative the WG will put out will be manufactured. So, all Morgans has to do is land on Elbaf. As an Underworld Emperor, Morgans is situated in the New World; considering how long it took the Straw Hats to get to (what we assume is) Elbaf, I think that's enough time for Morgans to reach Elbaf.
How was Vivi reintroduced in the New World? Jumping off of a ship:
Morgans goes to Elbaf to get a direct interview with Luffy, who is printing papers globally, and Vivi leaps off of Morgans ship once she realizes they're in Elbaf. This could set up a fun subplot of Morgans going after Vivi to keep her from leaking the truth about the Reverie, which would cause his reporting/reputation into question.