How to deny "not selected" option in Vue Select?

I have created select box like this:
JS:

Vue.component("v-select", VueSelect.VueSelect);

new Vue({
  el: "#app",
  data: {
    options: [
      { countryCode: "AU", countryName: "Australia" },
      { countryCode: "CA", countryName: "Canada" },
      { countryCode: "CN", countryName: "China" },
      { countryCode: "DE", countryName: "Germany" },
      { countryCode: "JP", countryName: "Japan" },
      { countryCode: "MX", countryName: "Mexico" },
      { countryCode: "CH", countryName: "Switzerland" },
      { countryCode: "US", countryName: "United States" }
    ],
  selectedCountry = null;
  someAnotherModel = null; // model for parent select box. country select box is depends on it.
  },
});

Html:

<v-select label="countryName" :options="options" v-model="selectedCountry"></v-select>

In some watcher of another select box I do this:

if (this.someAnotherModel == null) {
   this.selectedCountry = null; // if parent has not selected value, I nee clear also country select box, but isn't work
}

Can you help me fix my code please? My goals are:

  1. clear dynamically selected value and empty select box, I set to model null but this change is not reflected on selected value in select box
  2. also i have another question, I looking for it in documentation (http://sagalbot.github.io/vue-select/docs/), but I din’t found it. If I click on selected option, it will be unselected and select box will be clear. I want deny this behavior, and also set to select automatically some option if options array is not null.

Thanks in advice.