- Grace chose Frontier because of its route (key point).
She planned for the Vajra to hide within the Frontier and head towards the galactic center
There is something waiting for them in the galactic center
This point of the spoilers is quite interesting: what is "waiting for them" in the galaxtic center?!? In the center of galaxies there are black holes, but I don't think Grace&Friends are making this trip just to be destroyed by a black hole!
So it must be something else: in this regard it's usefull to remember what Grace&Friends replied to Luca in answer to his guess that Leon wanted the control over the galaxy through the fold quartz. Grace&Friends said (more or less): our target is what lies beyond that (=the galactic control). What may lie beyond the control over an entire galaxy? The control over the ENTIRE universe of course.
So there may be two things in the center of the galaxy:
1) a white hole, which is some kind of jump gate to other dimensions/galaxies. Maybe the Protoculture made something similiar to this
in their age thus leaving it behind.
2) some kind of device which allows instant travel between two points in the universe.
I think 1) is much more exciting than 2), but anyway both of the possibilities put the Macross storyline from the old galactic level to an entire new universe level. Macross Frontier may be the beginning of a new set of Macross series based on intergalactic travel and not only on intersteallar travel.
As an aside I think the Vajra may be beings put as guardians of whatever there is in the center of the galaxy by the Protoculture. Why? Because the Vajra, with their incredible potential, could have colonized the entire galaxy without problems, especially after the Protoculture's fall. Instead humans and, above all, Zentradis have never heard anything about them, which is quite strange because Zantradi's fleets are scattered all around the galaxy. This clearly shows the Vajra aren't an offensive force but a defensive force which is present only in the zones adjacent the galactic center.