<v-btn @click="explodeAll">{{exploded}}</v-btn>

<v-data-table
      :headers="headers"
      :items="items"
      item-key="name"
      ref="itemsTable"
      expand
    >
(..)
  <template slot="expand" slot-scope="props">
        <v-card flat>
          <v-card-text>Peek-a-boo!</v-card-text>
        </v-card>
  </template>
(..)
data () {
    return {
      exploded: false,
	    items: [..],
	    headers: [..],
	  }
},
methods: {
    explodeAll() {
      this.exploded = !this.exploded
      for (let i = 0; i < this.items.length; i += 1) {
        const item = this.items[i];
        this.$set(this.$refs.itemsTable.expanded, item.name, this.exploded);
      }
    }
},
(..)