Vertically center in viewport using CSS

What's that? Taking 8 years to get the answer to a problem is too much?

Well, better late than never!

You got really close to the solution. I'd do it with transform: translate():

#nav {
    position: fixed;
    right: 0;
    top: 50%;
    transform: translateY(-50%);
}

According to Can I use?, it is supported by everything except for IE8- and Opera Mini (which, to be honest, is a pretty good support).

I'd recommend you overkill it a bit and just add all of the vendor prefixes (just to make sure!):

#nav {
    position: fixed;
    right: 0;
    top: 50%;

    -webkit-transform: translateY(-50%);
       -moz-transform: translateY(-50%);
        -ms-transform: translateY(-50%);
         -o-transform: translateY(-50%);
            transform: translateY(-50%);
}

Here's a snippet to show it to you in action:

#nav {
    right: 0;
    top: 50%;
    position: fixed;
    -webkit-transform: translateY(-50%);
       -moz-transform: translateY(-50%);
        -ms-transform: translateY(-50%);
         -o-transform: translateY(-50%);
            transform: translateY(-50%);

    background-color: #ccc;
    padding: 20px;
}

<div id="nav">
    ABC<br/>
    DEFGH<br/>
    IJKLMNO<br/>
    PQRS<br/>
    TUVWXYZ
</div>

Hopefully it's still relevant to you! (who am I kidding, it's been 8 years)

you can use this as one of the solution.

   <style>
   #containter {
     height: 100vh; //vh - viewport height
     display: flex;
     flex-direction: column;
     align-items: center; 
     justify-content: center;
   }
   #content {}
  </style>

 <div id="containter">
  <div id="content">
    any text<br>
    any height<br>
    any content, for example generated from DB<br>
    everything is vertically centered
 </div>
</div>

If the item is set to position: fixed or position: absolute:

top: 50%; left: 50%; transform: translate(-50%, -50%)

If the item is set to position: relative, use:

margin-top: 50%; margin-left: 50%; transform: translate(-50%, -50%)

(More info at the source.)

Example:

Run the snippet and then resize this page (or rotate device). The box stays centered in the "snippet viewport".

.myContainer {  
  position: fixed;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  border: 5px solid RebeccaPurple;
}

.myThing {
  width: 100px;
  height: 100px;
  background-color: CornflowerBlue;
}

<div class="myContainer">
  <div class="myThing myContents">
  </div>
</div>