IMO, that's one of the good things about the series, everything is well foreshadowed and the series in general follows the law of conservation of detail, meaning whenever a "Chekov's gun" is shown it is "fired."
Grace was doing research alongside Dr. Mao Nome and Ranshe Mei. Grace has some idea to unify humanity in thought using implants and fold quartz. Mao and Ranshe disagree, meanwhile Ranka accidentally calls the Vajra (due to her mom having the V-type infection, passing the bacteria onto Ranka, but not in a lethal manner) and the Vajra, in confusion destroy the fleet. Grace cooks up some plan for revenge as she burns. Some time later, she finds Sheryl, who is now an orphan who has also has the V-type infection (whether or not these are Grace's doing is never explained). Grace, builds her up to be the idol that she becomes possibly hoping that she can replicate the effect that Ranka has on the Vajra. The Vajra were never really the object of Grace's revenge, but rather Ranshe and Mao - by achieving her goal which the two opposed, she would figuratively had her revenge and ended up as the "queen bitch of the galaxy" as well.
The destroying the Vajra bit only comes from Brera's actions in episode 9 which Grace probably ordered to keep the Frontier forces from learning more about the Vajra (as she had not yet allied/duped Leon). As for trying to kill of Sheryl, her plan at that point seemed to be to instigate war between the Vajra and Frontier and while research on hacking into the Vajra network (sans Ranka) was still ongoing. Finding Ranka was simply a bonus, allowing her to proceed at an accelerated pace.