[David Karol]

I got it. Instead of using the blocks manager, I inserted the ad tag directly into the FORUMDISPLAY and FORUMHOME templates, between the following lines:

<div id="sidebar_container"<vb:if condition="$show['sidebarposition'] == 'left'"> class="sidebarleft"</vb:if>>
	
		***AD TAGS GO HERE***
	        <br>
	
<a id="sidebar_button_link" href="#">

Note that I needed an extra line break at the end on the FORUMDISPLAY template only. Without it, the other sidebar blocks would show without any gap. This was not the case on the FORUMHOME template, where the line break seems to occur on its own.

Here's a screenshot of the result: