[Sarteck]

On the hook postbit_display_complete, you could add some PHP Code as a Plugin to test for the existence of a signature, and if it's absent, then put in your code.

For example,

PHP Code:

if (!$post['signature'])
{
  $this->post['signature'] == 'THIS IS A DEFAULT SIGGY!';
} 


Basically,

1. go to your AdminCP.
2. Expand "Plugins & Products"
3. Click "Add New Plugin"

On this page,

1. Leaving "Product" as vBulletin is just fine.
2. Choose postbit_display_complete as your hook.
3. Put in a descriptive title. "That Genius Sarteck's Solution for our Default Sigs!" springs to mind, but you can title it however you want.
4. Execution Order can be left at the default of 5.
5. Put in the PHP Code I provided.
6. Make sure that the Plugin is NOT active, at first.

Save and Reload.

Now, in another tab/window, open up a thread with someone who has no signature. Go back to your AdminCP (should still be on the Plugin you created since you saved and reloaded WITHOUT activating it). Now go ahead and choose "Yes" to activate it, then reload your thread, look, and if it works, great! If not, you come back here and say, "DAMN IT, SARTECK, YOU'RE A NINCOMPOOP FOR GIVING ME FALSE INSTRUCTIONS." And then we'll take it from there.

EDIT: P.S., setting the Plugin to "No" for it's activity is good if it doesn't work, too, so no one gets errors.