It is currently impossible to use the vueRouterAdditionalEventData option, because it is stored in public runtime config by the plugin and removed during serialization for the client bundle (see https://nuxt.com/docs/guide/going-further/runtime-config#serialization)
Reproduction:
// nuxt.config.ts
gtm: {
vueRouterAdditionalEventData: () => ({foo: 'bar'});
}
Result:
The additional data is not added to the trackView event payload