As discussed in https://vuejs.org/v2/guide/components.html#Using-v-model-on-...

  <Component v-model="foo">

is basically shorthand for

  <Component :value="foo" @input="foo=$event">

In particular, the child can't affect the value of the parent's "foo" attribute except by sending an event. This is in contrast to Angular 1 where changing "foo" in the child would also change it in the parent, which would often lead to confusing data flow cycles.