<div id="app1" v-bind:class="{class1: isOK, class2: class2IsOK}" v-on:mouseover="app1MouseOver" v-on:mouseleave="app1MouseLeave"></div>

.class1{    width: 200px;    height: 200px;    background: grey;}.class2{    border-radius: 100px;}

var app1 = new Vue({    el: "#app1",    data: {        isOK: true,        class2IsOK: false    },    methods: {        app1MouseOver: function(){            this.class2IsOK = true;        },        app1MouseLeave: function(){            this.class2IsOK = false;        }    }});