If they want to get revenge, Abra, I don't mind.
But perhaps they don't.
They don't in that utterance is included in the verbal substitution do or ellipsis? Is the full form
They don't want to get revenge
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Per Wikipedia, what you have is ellipsis: "words are omitted when the phrase needs to be repeated." The omitted words are "want to get revenge."
Substitution happens when a word is substituted for another word or words. That's not happening here. It would if your second sentence were "But perhaps they don't want that." In such a case, "that" would substitute for "to get revenge."