ТЕСТ

<div class="vh30">
    <div class="flexCenter">
        <h3>flex</h3>
    </div>
</div>

.vh30 {
    height: 30vh;
}    
.flexCenter {
    height: 100%;
    display: flex;
    flex-flow: column;
    align-items: center;
    justify-content: center;
}