After testing Vuex(3.0.1) in a Nativescript-vue (2.0) app I noticed that when using ModalViews or ListViews, $store is not available in the list items components or modal component. For now, use a StackLayout for lists and I pass the $store as a prop to the ModalView:

<!-- Instead of a ListView -->
<StackLayout class="list-group">
  <MyComponent v-for="item in items" :item="item" :key="item.id" class="list-group-item"/>
</StackLayout>

In MyComponent, the method to open the modal is like:

import ItemForm from '../forms/ItemForm'

export default {
  name: 'MyComponent',
  props: {
    item: { required: true, type: Object },
  },
  methods: {
    openForm () {
      const props = {item: this.item, store: this.$store} // props for ItemForm
      this.$showModal(ItemForm, { props })                // Show ItemForm as a ModalView
    },
  },
}