SOURCE

new Vue({
    el: '#app',
    data: {
        color: 'gray',
        width: 100
    },
    computed: {
        myStyle: function() {
            return {
                backgroundColor: this.color,
                width: this.width + 'px'
            }
        }
    }
})

<div id="app">
    <div class="demo" :style="{backgroundColor: color}"></div>
    <div class="demo" :style="myStyle"></div>
    <div class="demo" :style="[myStyle, {height: width + 'px'}]"></div>
    <hr>
    <input type="text" v-model="color">
    <input type="text" v-model="width">
</div>

.demo {
    width: 100px;
    height: 100px;
    background-color: gray;
    display: inline-block;
    margin: 10px;
}

.red {
    background-color: red;
}

.green {
    background-color: green;
}

.blue {
    background-color: blue;
}