There was a reason Vaan made the debris field his battleground. It had plenty of obstacles to hide him from the pure earth elemental worm, even if for a brief moment.
Furthermore, it was already sufficiently far from Pangea.
Although it seemed like Vaan planned to lure the pure earth elemental worm all the way into the vastly larger region of space, the asteroid field, it was already established that it was too far for him.
However, the most important factor was the unknown danger in the asteroid field.
Considering the pure earth elemental worm could generate spacequakes, there was no telling whether similar life-destroying existences were lurking in the depths of the asteroid field.
In fact, there was even a good chance that such existences caused a group of rogue asteroids to head toward Pangea.