<ion-icon :icon="add"> is the correct usage. This is documented in the quickstart guide (Ionic Vue Quickstart - Ionic Documentation), but looks like we did not update the ion-fab example to document the correct ion-icon usage. I will put up a PR to fix this. Thanks!
edit: PR is up: docs(fab): document correct ion-icon usage for vue by liamdebeasi · Pull Request #23352 · ionic-team/ionic-framework · GitHub