Vertical align not working on inline-block

It doesn't work because vertical-align sets the alignment of inline-level contents with respect to their line box, not their containing block:

This property affects the vertical positioning inside a line box of the boxes generated by an inline-level element.

A line box is

The rectangular area that contains the boxes that form a line

When you see some text which has multiple lines, each one is a line box. For example, if you have

p { border: 3px solid; width: 350px; height: 200px; line-height: 28px; padding: 15px; }

<p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Donec a diam lectus. Sed sit amet ipsum mauris. Maecenas congue ligula ac quam viverra nec consectetur ante hendrerit. Donec et mollis dolor. Praesent et diam eget libero egestas mattis sit amet vitae augue. Nam tincidunt congue enim, ut porta lorem lacinia consectetur.</p>

If there is only a single line, like in your case, it's also a line box

Using vertical-align: middle centers .content vertically inside that line box. But the problem is that the line box is not vertically centered inside the containing block.

If you want to center something vertically at the middle of the containing block see How to align text vertically center in div with CSS?

Vertical-align on inline / inline-block element, images, text... align element together, not with parent.

Usage example: align smiley image in a text

you can cheat by adding a 0px width, 100% height pseudo-element

#wrapper {
    border: 1px solid black;
    width: 200px;
    height: 200px;
    vertical-align: middle;
}
#wrapper:after{
    content: '';
    display: inline-block;
    width: 0px;
    height: 100%;
    vertical-align: middle;
}
#content {
    border: 1px solid black;
    display: inline-block;
    vertical-align: middle;
}

<div id = 'wrapper'>
    <div id = 'content'> content </div>
</div>

It work for me by using vertical-align:text-top;

<div class="box">
    Lorem ipsum dolor sit amet, consectetuer adipiscing elit, sed diam nonummy nibh 
</div>
<div class="box">
    Lorem ipsum dolor sit amet, diam nonummy nibh 
</div>
<div class="box">
    Lorem ipsum dolor sit amet, consectetuer adipiscing elit, sed diam nonummy nibh 
</div>

.box {
   display: inline-block;
   width: 20%;
   margin: 5px;
   padding:10px;
   border-top: 5px solid #000000;
   vertical-align: text-top;
}

jsfiddle