Vue.use(VueRouter)

const contactGuard = function(to, from, next) {
  if (to.name === 'contact') {
    this.$root.$emit('modal', true)
    next(false)
  } else {
    next()
  }
}

const routes = [{
    path: '/',
    name: 'home',
    component: {
      template: `<div><p>Home Page</p></div>`,
      beforeRouteLeave: contactGuard
    }
  },
  {
    path: '/about',
    name: 'about',
    component: {
      template: `<div><p>About Page</p></div>`,
      beforeRouteLeave: contactGuard
    }
  },
  {
    path: '/contact',
    name: 'contact',
    beforeEnter: (to, from, next) => {
      if (from.path) {
        // we're coming from another page
        this.$root.$emit('modal', true)
        next(false)
      } else {
        // we've directly navigated to /contact
        next({
          replace: true,
          path: '/',
          query: {
            contact: true
          }
        })
      }
    }
  }
]

const router = new VueRouter({
  routes
})

router.beforeEach((to, from, next) => {
  next(vm => {
    if (to.query.contact) {
      vm.$root.$emit('modal', true)
    }
  })
})

new Vue({
  el: '#app',
  router,
  data() {
    return {
      modal: false
    }
  },
  mounted() {
    this.$on('modal', show => {
      this.modal = show
    })
  }
})

.modal-mask {
  position: fixed;
  z-index: 9998;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background-color: rgba(0, 0, 0, .5);
  display: table;
  transition: opacity .3s ease;
}

.modal-wrapper {
  display: table-cell;
  vertical-align: middle;
}

.modal-container {
  width: 300px;
  margin: 0px auto;
  padding: 20px 30px;
  background-color: #fff;
  border-radius: 2px;
  box-shadow: 0 2px 8px rgba(0, 0, 0, .33);
  transition: all .3s ease;
  font-family: Helvetica, Arial, sans-serif;
}

.modal-header h3 {
  margin-top: 0;
  color: #42b983;
}

.modal-body {
  margin: 20px 0;
}

.modal-default-button {
  float: right;
}


/*
 * The following styles are auto-applied to elements with
 * transition="modal" when their visibility is toggled
 * by Vue.js.
 *
 * You can easily play with the modal transition by editing
 * these styles.
 */

.modal-enter {
  opacity: 0;
}

.modal-leave-active {
  opacity: 0;
}

.modal-enter .modal-container,
.modal-leave-active .modal-container {
  -webkit-transform: scale(1.1);
  transform: scale(1.1);
}

<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/css/bootstrap.min.css" integrity="sha384-MCw98/SFnGE8fJT3GXwEOngsV7Zt27NXFoaoApmYm81iuXoPkFOJwJ8ERdknLPMO" crossorigin="anonymous">
<script src="https://code.jquery.com/jquery-3.3.1.slim.min.js" integrity="sha384-q8i/X+965DzO0rT7abK41JStQIAqVgRVzpbzo5smXKp4YfRvH+8abtTE1Pi6jizo" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.3/umd/popper.min.js" integrity="sha384-ZMP7rVo3mIykV+2+9J3UJ46jBk0WLaUAdn689aCwoqbBJiSnjAK/l8WvCWPIPm49" crossorigin="anonymous"></script>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/js/bootstrap.min.js" integrity="sha384-ChfqqxuZUCnJSK3+MXmPNIyE6ZbWh2IMqE241rYiqJxyMiZ6OW/JmZQ5stwEULTy" crossorigin="anonymous"></script>
<script src="https://unpkg.com/vue/dist/vue.js"></script>
<script src="https://unpkg.com/vue-router/dist/vue-router.js"></script>

<div id="app">
  <nav class="navbar navbar-expand-lg navbar-light bg-light">
    <a class="navbar-brand" href="#">Foobar</a>
    <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
    <span class="navbar-toggler-icon"></span>
  </button>

    <div class="collapse navbar-collapse" id="navbarSupportedContent">
      <ul class="navbar-nav mr-auto">
        <li class="nav-item">
          <router-link class="nav-link" :to="{ name: 'home' }">Home</router-link>
        </li>
        <li class="nav-item">
          <router-link class="nav-link" :to="{ name: 'about' }">About</router-link>
        </li>
        <li class="nav-item">
          <router-link class="nav-link" :to="{ name: 'contact' }">Contact</router-link>
        </li>
      </ul>
    </div>
  </nav>
  <div class="container">
    <router-view></router-view>
  </div>
  <transition name="modal">
    <div class="modal-mask" v-if="modal">
      <div class="modal-wrapper">
        <div class="modal-container">

          <div class="modal-header">
            <slot name="header">
              default header
            </slot>
          </div>

          <div class="modal-body">
            <slot name="body">
              default body
            </slot>
          </div>

          <div class="modal-footer">
            <slot name="footer">
              default footer
              <button class="modal-default-button" @click="$root.$emit('modal', false)">
                OK
              </button>
            </slot>
          </div>
        </div>
      </div>
    </div>
  </transition>
</div>