Okay so I use rejoin in quotations because she really is one already.
The thing is, this chapter we saw all the Straw Hats and some others like Croc and Vivi responding to news of something Luffy did.
Now, this is 3 weeks after the war….likely after Luffy and Rayleigh met. Luffy had wanted to go see his crew right away, but Rayleigh warned him that reuniting now means repeating the same thing all over again. Of course without Roger's first mate there to guard him and an undercover revolutionary there to save them, next time it would mean their death.
I think whatever Luffy did was meant to send a message to his crew to sit tight, and take their time reuniting...get stronger first. The others will train and take advantage of the islands they're on to get stronger and after time passes (maybe a year or two...no more than three though), they will reunite.
One thing to consider is the change of the war and the increasing focus on the theme of freedom and oppression by nobles, celestial dragons, etc.
This brings up the subject of Nefertari Vivi. She stayed behind in Alabasta for the sake of her country. However, she also left Alabasta once before for the sake of her country. She left to stop Crocodile's plans.
Alabasta is under the control of the World Government and increasingly at the risk of attack by Pirates with the rising tide of unrest due to the death of Whitebeard and Ace at Marineford.
So what if protecting Alabasta once again meant leaving?
If Luffy's message also included the idea of where to meet...Vivi also read that message. What if Vivi and Karoo also make their way to this location to rejoin the crew, leaving behind Alabasta for a few years to save Alabasta, the same way she did before. Just as Crocus became a pirate for 3 years, so too may the Desert Princess Vivi.
After all, she is a Straw Hat, so she may understand Luffy's message as well as the others.
This is of course all speculation, but the way it was said she read the paper and thought about it and then returned to her room...what if Vivi used this time to train and get stronger as well? Then upon reuniting, she too might be strong enough to fight alongside the others.
Her dream? Easy, freeing Alabasta and protecting her kingdom!