我用了一个比较tricky的方式解决，但并不是真正的解决方案。

var joinRoom = $('#joinRoom'),joinRoomLabel=$('#joinRoomLabel'),curRoomId,
onlineAlert=$('#onlineAlert'),offlineAlert=$('#offlineAlert'),chatLeft=$('#chatLeft');
Vue.component('room',{
  props:['message'],
  methods:{
    selectRoom:function(){

            chatLeft.empty();
            curRoomId = this.message._id;
            joinRoomLabel.text(this.message.roomName);

            joinRoom.modal('toggle');
            socket.emit('online', {user: from, curRoomId: curRoomId});
            if(this.message.roomChat=='secret'){
                vRoom.getUsers();
            }else{
                vRoom.getpUsers();
            }
    }
  },
  template: '<p class="room"><p class="room-header"><p class="room-header-back"></p><p class="room-header-content"><p>{{message.roomName}}</p><p><span class="glyphicon glyphicon-plus-sign" v-on:click="selectRoom"></span></p></p></p><p class="room-body"><img src="/public/imgs/0.png"><p class="room-text">{{message.roomContent}}</p></p><p class="room-footer"><p class="room-footer-back"></p><p class="room-footer-content"><p class="room-peos">参与：<span>{{message.roomJoinPeos}}人</span></p><p><span class="glyphicon glyphicon-heart"></span></p> </p></p></p>'
});
var vRoom = new Vue({
  el:'#joinRoom',
  data:{
    users:[],
    messageBody:'',
    roomChat:'',
    roomJoinPeos:''
  },
  methods:{
    upUsers:function(data){
      this.users = [];
      var us = [];
      for(var u in data){
        us.push(u);
      }
      if(us.length==3){
            alert("房间已满,但在同一房间的人会知道您曾经尝试进入该房间");
            joinRoom.modal('toggle');
            this.closeRoom();
            return;
        }
      this.users = us;
    },
    getUsers:function(){
      $.get('/room/users/'+curRoomId,function(data){
        this.upUsers(data);
      }.bind(this));
    },
      uppUsers:function(data){
          this.users = [];
          var us = [];
          for(var u in data){
              us.push(u);
          }
          this.users = us;
      },
      getpUsers:function(){
          $.get('/room/users/'+curRoomId,function(data){
              this.uppUsers(data);
          }.bind(this));
      },
    sendMessage:function(){
        if(this.messageBody){
          chatLeft.append(' <p class="chatLeftOwenr"><p class="chatLeftOwenrPeo">'+from+'</p><p class="chatLeftOwenrContent">'+this.messageBody+'</p></p>');
          socket.emit('say',{toObj:'all',messageBody:this.messageBody,curRoomId:curRoomId,user:from});
          this.messageBody='';
        }
    },
    closeRoom:function(){
        socket.emit('offline',{curRoomId:curRoomId,user:from});
        curRoomId = '';
    }
  }
})

room 这个组件没有被引用过吧..

触发事件是 this.$emit啊