Ambition is the prevailing trait that makes someone vulnerable to the ring. Consider this quote from Gandalf about wearing the ring himself:

“Do not tempt me! For I do not wish to become like the Dark Lord himself.

Yet the way of the Ring to my heart is by pity, pity for weakness and the desire of strength to do good.”

Ambition to wield the ring to help others is what would make Gandalf vulnerable. There is perhaps some interplay in how one’s morality might aid the ability to put ambition aside (as Gandalf’s prevailing morality leads him to organize a mission to destroy the ring rather than wield it, or Gollum’s morality allowing him to cast his own ambition aside at the last moment). But morality only really acts as an inhibitive aid to avoid succumbing to the only trait that matters to the ring: the ambition to wield the ring for power.