You are right, Bush didn't answer the question.
Given that there were no arrests up to that point and given that the President couldn't guarantee that there would be no future vigilante actions, as chief law enforcement officer it would be inappropriate for Bush to answer the question in the following two ways;
(2)Anybody who is at the border now and anybody (including) the Minutemen who will soon be at the border are not vigilantes and will not be vigilantes when they get to the border.
Regarding vigilantism at the border, LE has one task. Arrest and prosecute anybody who has committed a vigilante act.
Since there hadn't been any arrests and since the President can't predict the future, instead of answering the setup question, the President went on to do his job and say what he wanted to see at the border and what he didn't want to see at the border.
BTW, is your keyboard br0ken?
LE DOES have other responsibilities at and around the border. They have the responsibility to protect the citizens from trespass by illegals. Any vigilante actions in the past (not necessarily associated with the border and definitely not the KKK) have been associated with a lack of LE presence/enforcement.
W wouldn't even have to be concerned with any vigilante action if he had been doing his job of enforcing existing laws regarding illegal border crossers. All of this really does come down to W's opinion of the existing laws and his responsibilities to enforce same.
Not answering a legitimate question is weaseling. Using the word vigilante is inflamatory.