There is no need to use a hack for this if you wish to add it directly to the template, simply doing the following will work:
<if condition="$vbulletin->userinfo['posts'] > 10">
Hidden text
</if>
Obviously change the number 10 with the minimum amount of posts required to view the text. If you want to do this inside a post, you will need to install a hide hack.
Hope this helps