"Vestibular extension" is a surgical procedure, not a complication. In this context, the text refers to "vestibulización" as the process whereby the root of the tooth is displaced buccally (toward the vestibule of the mouth).
You will find many, many references to "vestibular displacement" or "buccal displacement" in the literature, or via a quick Google search.

Incidentally, "vestibular" is usually best avoided as an anatomical term of location in English, as it it imprecise; "labial" (for the anterior teeth/those in the aesthetic zone) or "buccal" (for all other teeth) is preferable.

Suerte :)