vBulletin WILL display signatures to GUESTS normally. If your forum isn't showing them to guests then you or another admin did something to stop that. This could be a mod, a plugin, or a template edit... It's hard to know for sure- that is all the help I can give you without knowing more.
In your postbit or postbit_legacy template, whatever you use, the code for the signature should be:
<if condition="$post['signature']">
<!-- sig -->
<div>
__________________<br />
$post[signature]
</div>
<!-- / sig -->
</if>
If you see an additional <IF> statement or a different <IF> change it to the default code above and see if that works.