[Vuejs]-How do I create a mobile navbar using bootstrap 4 with Vue?

Taking a look at the Bootstrap-Vue docs, it seems that the issue in there (and therefore on your code as well) is that the markup places the b-navbar-toggle before the b-navbar-brand, so when the size of the screen is reduced the button appears before the brand, causing the result you see. Simply move the b-navbar-toggle to be just before the opening tag of the collapse section and you will get the correct result.

<b-navbar toggleable="md" type="dark" variant="info">
  <b-navbar-brand href="#">NavBar</b-navbar-brand>

  <!-- Put the toggle here after the brand -->
  <b-navbar-toggle target="nav_collapse"></b-navbar-toggle>

  <b-collapse is-nav id="nav_collapse">

    <b-navbar-nav>
      <b-nav-item href="#">Link</b-nav-item>
      <b-nav-item href="#" disabled>Disabled</b-nav-item>
    </b-navbar-nav>

    <!-- Right aligned nav items -->
    <b-navbar-nav class="ml-auto">

      <b-nav-form>
        <b-form-input size="sm" class="mr-sm-2" type="text" placeholder="Search"/>
        <b-button size="sm" class="my-2 my-sm-0" type="submit">Search</b-button>
      </b-nav-form>

      <b-nav-item-dropdown text="Lang" right>
        <b-dropdown-item href="#">EN</b-dropdown-item>
        <b-dropdown-item href="#">ES</b-dropdown-item>
        <b-dropdown-item href="#">RU</b-dropdown-item>
        <b-dropdown-item href="#">FA</b-dropdown-item>
      </b-nav-item-dropdown>

      <b-nav-item-dropdown right>
        <!-- Using button-content slot -->
        <template slot="button-content">
          <em>User</em>
        </template>
        <b-dropdown-item href="#">Profile</b-dropdown-item>
        <b-dropdown-item href="#">Signout</b-dropdown-item>
      </b-nav-item-dropdown>
    </b-navbar-nav>

  </b-collapse>


</b-navbar>