[King Kovifor]

I believe that if "Reputation" is enabled in ACP -> vBulletin Options -> User Reputation Options, that the rep power will display automatically. If you wanted to use "Powerful" and so on, you could use <if>'s in the template. Something along these lines:

<if condition="$post['rep_power'] > 50"><span style="color: red;">Powerful</span></if>

Of course, I'm not sure on the $post['rep_power'], since I don't use reputation, but you should be able to look at your HTML through the postbit(_legacy) template and find the reputation power value.

This should do what you are looking for.