test2.vue

<template>
    <input v-model="foo.bar"> // this will work
    <input v-model="foo.bar.baz"> // this will work
</template>

<script>
export default {
  ....
  data() {
    return {
      foo: { 
       bar: { baz: undefined } 
      }
     };
  }
}
</script>