[Vuejs]-Vue.js textbox disappears with v-model keyword

You mentioned you are trying to reuse / modify someone else’s code. I believe you are trying to do some kind of “inheritance” by using Vue.extend.

If so, please note that inheritance is not recommended. For details, here is the link to discussion topic: https://github.com/vuejs/Discussion/issues/354

That discussion was around Vue 1.0 version, but I believe it is still relevant for Vue 2.0.

As mentioned in that page, if you are working in a team and sharing code / components, then you may use mixins instead. You will find more details in the docs: http://vuejs.org/guide/mixins.html