1) Using your vBulletin administrator login, go to the admin panel and click on the Style Manager menu option (under the Styles & Templates heading):

2) Click on the Expand Templates button:

3) Find Forum Home Templates in the template list, and double-click it:

4) Double click on FORUMHOME:

5) Within the template contents, find the string

<if condition="$show['loggedinusers']">

Place your cursor right before it, and paste the following text:

<!-- flashchat users -->
<tbody>
<tr>
<td class="thead" colspan="2">Flashchat users</td>
</tr>
</tbody>
<tbody>
<tr>
<tr>
<td class="alt2" colspan="2">
<iframe frameborder="0" height="100" src="chat/info_embedded.php" style="width: 100%; margin: 0; padding: 0; border: 0; overflow: hidden;" scrolling="no" border="0" allowtransparency="true"></iframe>
</td>
</tr>
</tbody>
<!-- end flashchat users -->

You can tweak the height parameter of the <iframe> tag to adjust the height of the userlist.

6) Click on the Save button

Now, your forum home page should look like this:

Now, your forum's home page should look like this: