给子容器一个相对位置，给中间段落一个绝对位置，然后给中间段落一个上下边距50vh：

.container {
  display:flex;
  align-items: center;
  height: 300px;
  background-color: red;

}

.child-container {
  display: flex;
  flex-direction: column;
  justify-content: space-evenly;
  background-color: purple;
    position: relative;
}

.big{
  line-height:40px;
}

.bigger{
  line-height:80px;
}

.center {
    position: absolute;
    margin-top: 50vh;
    margin-bottom: 50vh;
}

<div class="container">
<div class="child-container">
  <p class="big">Lorem ipsum</p>
  <p class="center">Center this</p>
  <p class="bigger">Lorem ipsum</p>
</div>
</div>

假设您的意思是水平居中，那么您只需要使用flex-grow扩展子容器，并对您选择的子容器使用align-self：

.container {
    display: flex;
    align-items: center;
    height: 300px;
    background-color: red;
  }

  .child-container {
    display: flex;
    flex-direction: column;
    justify-content: space-evenly;
    background-color: purple;
    flex-grow: 1;
    align-items: flex-start;
  }

  .child-container * {
    background-color: greenyellow;
  }

  .big {
    line-height: 40px;
  }

  .bigger {
    line-height: 80px;
  }

  .center {
    align-self: center;
  }

<div class="container">
<div class="child-container">
  <p class="big">Lorem ipsum</p>
  <p class="center">Center this</p>
  <p class="bigger">Lorem ipsum</p>
</div>
</div>