Vindicta was already there. If you're going to the Heart of Gielinor, it's generally a good idea to bring the guy who has been fighting in your name there for a few months solid.
As for Zaros' negligence towards Wahisietel and Akthanakos, I believe Zaros is actively avoiding them so as to avoid restoring his influence on them.
Wahisietel has proven very capable at acting independently, and while Akthanakos hasn't really accomplished anything he has been trying. Zaros might be deliberately not summoning them so as to not put them back under the influence of his curse so that they can continue acting independently.
If this is true, it's a very good idea, though it is negligent of him not to convey that message to them. He might've told Azzy about it, expecting him to tell them, but Azzy being Azzy...
If only he'd tell the World Guardian why he isn't including them in his plans we could at least ease Wahi and Akthanakos' minds.
"Abscondita est in Astra."
