One of the first steps in our migration to Vue 3 will be to upgrade the version of Vue in MediaWiki from Vue 2.6 to the migration build of Vue 3. This migration build is mostly compatible with Vue 2, but it doesn't provide compatibility for certain edge cases or for features that are undocumented or not officially supported. The Vue code in Wikibase (including its submodules, like termbox, and the WikibaseLexeme extension which doesn't appear to have its own Phabricator project) needs to be tested against the Vue 3 migration build and updated to fix any errors.
See the "test plan" section on T289019: Test Vue 3 migration build with extensions/skins using Vue for more detail. The outcome of this task should be that Wikibase runs without errors both under Vue 2.6 and under the migration build of Vue 3. These changes should ideally be minor; actually migrating to Vue 3 should be done later, since it's more work and will lead to incompatibility with Vue 2.