If you're using postbit_legacy (like I am) this template conditional will work.
Find in postbit_legacy:
Code:
<if condition="$post['signature']">
Replace with:
Code:
<if condition="$post['signature'] AND !$vboptions['shownsigs'][$post[userid]] AND $vboptions['shownsigs'][$post[userid]] = true">
Works like a charm for me. Note, I did not write this, I found it on
vBulletin.com after looking here first.