First, put your site into debug mode. You should do this on a test site, but if you don't have one then the this article might be of interest:
www.vbulletin.org/forum/showthread.php?t=265999 because you shouldn't really have your site in debug mode for everyone.
When you do that, then in the admincp, new links will appear. You can add a new settings group, or you can just add a setting to an existing group. You need to pick a unique value for vaname. That can't be changed, so if you need to change it later you need to delete your setting and add a new one. The rest of the settings can be edited as needed.
Lets say you add a setting with 'my_text' as the varname (probably not a good choice in real life, but just as an example). Then whatever is entered there will be in $vbulletin->options['my_text']. [S]Unfortunately, that's not one of the variables that is automatically registered to a template[/S], Edit: actually I was wrong, I don't know why I believed that, but the options are registered automatically as vboptions. So you don't need a plugin, and I've changed the rest of this post to reflect that.
Then in template template_name you can use either {vb:raw vbotions.my_text} or {vb:var vboptions.my_text} (using var will encode any special html chars like < and > so that they'll appear in the output, but doing that would make it impossible to include html in your field, so choose as appropriate).
Also if you want, in an if condition you can use $my_text, like <vb:if condition="$vboptions[my_text] == 'something'">...