Flex items not centering vertically

Question

I'm trying to center vertically the content with flex boxes without success. I don't like to use position or translate because it is not a fixed size. What is wrong with my code?

.row-centered {
    display: flex;
    flex-wrap: wrap;
    justify-content: center;
    align-items: center;
}

.col-centered {
    display: flex;
    flex-direction: column;
    align-self: flex-start;
    float: none;
    margin-right: 0 auto;
}

.us-container{
  display: flex;
  justify-content: center;
  align-items: center;
  vertical-align: middle;
  resize: both;
  overflow: auto;
}

.us-container div{
  resize: both;
  overflow: auto;
}

    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">

<section id="us" class="container-fluid">
        <div class="row row-centered  us-container">
            <div class="col-sm-8 col-md-8 col-xs-8 col-centered ">
                <p class="text-justify">blah blah blah blah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blah </p>
                <h3 class="text-center">blah blah blah</h3>
            </div>
            </div>
    </section>

Answer 1

Your flex container has no extra height. The only height is the height of the content. Therefore, there is no space for vertical centering.

First step, add some height:

.row-centered {  
  display: flex;
  height: 100vh;
}

Second step, remove unnecessary rules, some of which are preventing vertical alignment.

.col-centered {
    display: flex;
    flex-direction: column;
   /* align-self: flex-start;
    float: none;
    margin-right: 0 auto; */
}

.row-centered {
  display: flex;
  height: 100vh;
}
.col-centered {
  display: flex;
  flex-direction: column;
}
.us-container {
  display: flex;
  justify-content: center;
  align-items: center;

}

<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" rel="stylesheet" />
<section id="us" class="container-fluid">
  <div class="row row-centered us-container">
    <div class="col-sm-8 col-md-8 col-xs-8 col-centered ">
      <p class="text-justify">blah blah blah blah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah
        blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blahblah blah blah</p>
      <h3 class="text-center">blah blah blah</h3>
    </div>
  </div>
</section>

Answer 2

Add height: 100% to html, body, .container-fluid and make .container-fluid also be a flex container as shown below.

html, body, .container-fluid {
    height: 100%;
}

.row-centered, .container-fluid {
    display: flex;
    flex-wrap: wrap;
    justify-content: center;
    align-items: center;
}

.col-centered {
    display: flex;
    flex-direction: column;
    align-self: flex-start;
    float: none;
    margin-right: 0 auto;
}

.us-container{
  display: flex;
  justify-content: center;
  align-items: center;
  vertical-align: middle;
  resize: both;
  overflow: auto;
}

.us-container div{
  resize: both;
  overflow: auto;
}

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">

<section id="us" class="container-fluid">
        <div class="row row-centered  us-container">
            <div class="col-sm-8 col-md-8 col-xs-8 col-centered ">
                <p class="text-justify">blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah </p>
                <h3 class="text-center">blah blah blah</h3>
            </div>
            </div>
    </section>